The typical value - and there is no typical actually when it comes to hormones - for SHBG that we see on the The Peak Testosterone Forum is probably 28-32. But there is a wide range above and below this. So what if you're high on SHBG? Is higher SHBG good or bad for you male health and how does it affect testosterone?
Well, let's first discuss what SHBG does to testosterone: it binds with it and makes it unusable for all practical purposes. So, in general, the more SHBG that you have, the less free testosterone that you have, because free testosterone is the testosterone that is not bound to albumin or SHBG and is most ready to do actual work on your tissues.
NOTE: Actually, the testosterone that is bound to albumin is weekly bound and can also be used to do easy work on the tissues, but the conclusions are still the same.
What this effectively does is turn SHBG into a kind of rheostat for the amount of "working testosterone" that you have. If you dial SHBG up, your free T goes down. If you dial SHBG down, your free T goes up. And it should be pointed out that in the course of a man's life, his SHBG levels will double on average.  That's right - that's a big increase and certainly one of the cause of andropause symptoms in senior men.
Now it seems like a no-brainer that you'd actually want to always lower your SHBG then, right? Well, unfortunately, both high and low SHBG are very often associated with ugly medical conditions. Just as low SHBG is not really something to be proud of, a fact I cover in my link on The Causes and Effects of Low SHBG, neither is high. Consider these medical conditions associated with elevated SHBG:
1. Fracture Risk. One study of senior men showed an augmented fracture risk to be associated with increasing SHBG.  This would, presumably, be from SHBG's ability to also bind to both testosterone and estradiol, leaving less of both in circulation. This probably applies only to men with lower estradiol levels. 
2. Liver Disease. Researchers have noted that liver diseases, such as cirrhosis or NAFLD (non-alcoholic fatty liver disease) are associated with increase SHBG. Since the liver is the source of most of our SHBG, wouldn't an injured liver actually produce less SHBG? It turns out that a damaged liver actually allows the build up of estrogens and these, in turn, stimulate SHBG production.
3. Alcoholism and Cirrhosis of the Liver. Heavy drinking can damage the liver and lead to cirrhosis. Once it reaches this point, SHBG can be elevated. 
4. Elevated Estradiol. SHBG synthesis is accelerated by estradiol, the "chief estrogen." If estradiol levels get too high, through weight gain, medications or other factors, SHBG can rise accordingly.
5. Clomid. Many men are on Clomid for fertility-preserving reasons. In a man with secondary hypogonadism, which is the most common case, testosterone can be boosted significantly as well. Unfortunately, Clomid can significantly raise SHBG in some men. One study found that "SHBG concentration rose from 38.1 +/- 18.3 to 54.3 +/- 16.0 nmol/l." 
6. Hyperthyroidism. Hyperthyroidism will raise SHBG according to a couple of studies. And that's no surprise as research has shown that SHBG is related to levels of T3 and T4. 
7. Low Magnesium or Zinc. This two minerals affect just about everything and, if you are dificient, supplementation can lower your SHBG a bit. (Don't take too much zinc. See my page on The Potential Dangers of Zinc for more information.)
8. Low DHEA? Taking DHEA can lower SHBG.  So perhaps being low in DHEA could lead to elevated SHBG levels?
9. Many Other Medical Conditions. Hyperthyroidism, HIV infection, malnutrition, some drugs such as anticonvulsants and malnutrition can all raise SHBG. (You might want to check atrophic gastritis if you are showing signs of the latter.)
Similarly, it might seem that elevated SHBG might always be bad. However, this is not always the case: slightly elevated SHBG levels are associated with good health. Here are some examples:
1. Reduction in Prostate Cancer Risk. Several studies have found that as SHBG goes up, prostate cancer risk goes down. For example, consider the statement of this abstract: "When hormone and SHBG levels were adjusted simultaneously, a strong trend of increasing prostate cancer risk was observed with increasing levels of plasma testosterone (ORs by quartile = 1.00, 1.41, 1.98, and 2.60 [95% CI = 1.34 5.02]; P for trend = .004), an inverse trend in risk was seen with increasing levels of SHBG (ORs by quartile = 1.00, 0.93, 0.61, and 0.46 [95% CI = 0.24 0.89]." 
2. Lower Insulin Levels. There is evidence that elevated insulin lowers SHBG and thus higher values of SHBG can indicate lower levels of insulin.  (This is ironic, because testosterone lowers insulin and SHBG binds testosterone. So one pathway must be powerful generally than the other.) The above study explained that insulin's SHBG-lowering action was shown in vitro, i.e. in the "test tube", but subsequent studies have shown it in vivo as well, i.e. on human subjects and male ones at that.  This is why diabetics tend to have lower SHBG values.
3. Decreased Diabetes and Prediabetes (Metabolic Syndrome). Some fairly recent study work has shown that, undoubtedly due to #2, a strong inverse relationship between diabetes/prediabetes and SHBG. One set of authors wrote clearly: "Low total testosterone and SHBG levels independently predict development of the metabolic syndrome and diabetes in middle-aged men." 
Thus, it becomes obvious that SHBG probably has a "safe range" for most men, where it signals improved mortality and health outcomes.
If you have elevated SHBG, talk to your doctor of course. But here are some additional ideas to help you lower your levels naturally. Again, remember that overly elevated levels will rob you of free testosterone potentially:
1. Correct Any Medical Issues. Step #1 should be to correct any medical issue - see the above - that has lead to excessively high SHBG. Getting to the root cause is always a good idea!
2. Protein. Animals studies have shown that increasing protein intake can lower SHBG. 
3. Fish Oil. Omega-3's can help with inflammation, depression, mood and cardiovascular outcomes according to some experts, so why not with free testosterone as well? (CAUTION: There is a recent study showing fish oil increases prostate cancer risk though.)
4. Vitamin D. According to one large study on men, the higher the Vitamin D, the higher the testosterone. And the good news does not stop there: the higher the Vitamin D, the lower the SHBG according to this same study. 
5. Dietary Fat. One (small) study on six men found that if the men ate a higher fat diet, > 100 grams fat/day, their SHBG dropped. This may seem like a good thing, but remember that high fat diets are used by researchers to induce insulin resistance.  A smiliar study echoed the same result and suggested that fiber may play a role as well. 
6. Stinging Nettle. Stinging Nettle is a proven SHBG lowerer and it does so by affecting the liver. I cover this in much more detail in my link on Free Testosterone.
7. HRT (Testosterone Therapy). I consider HRT to be a natural treatment option, assuming that testosterone is kept within physiological ranges. And exogenous testosterone will generally lower SHBG (along with many other benefits).
8. DHEA. DHEA can lower SHBG as shown in a couple of studies.  Anecdotally, one of our forum posters heard from his doctor that low DHEA can lead to elevated SHBG.  Interestingly, this doctor did not recommend supplemental DHEA but rather to lower stress. However, some of the adrenal fatigue docs treat their patients with this hormone when one is deficient. That said, DHEA supplementation is tricky and some of it will always convert to estradiol. It is somewhat of a crapshoot knowning whether the DHEA will turn properly into testosterone or estradiol. DHEA can also lower cortisol, which is not always desireable in every case. Adding to the confusion is the fact that some practitioners give 10 mg to their male patients when needed and still others 50 mg. So dosage can be vague and controversial.
2) The Journal of Steroid Biochemistry and Molecular Biology, Jun 1995, 53(1-6): "Interrelations between sex hormone-binding globulin (SHBG), plasma lipoproteins and cardiovascular risk"
3) The Journal of Clinical Endocrinology & Metabolism, Jul 1 1996, 81(7):2515-2519, "Acute and chronic regulation of serum sex hormone-binding globulin levels by plasma insulin concentrations in male noninsulin-dependent diabetes mellitus patients"
4) Diabetes Care, May 2004, 27(5):1036-1041, "Testosterone and Sex Hormone Binding Globulin Predict the Metabolic Syndrome and Diabetes in Middle-Aged Men"
5) Bone Miner Res, 2008 Oct, 23(10):1552-60, "Older men with low serum estradiol and high serum SHBG have an increased risk of fractures"
6) The Journal of Clinical Endocrinology & Metabolism, Dec 1 2006, 91(12):4764-4766, "Sex Hormone Binding Globulin: Inhibitor or Facilitator (or Both) of Sex Steroid Action?"
7) Journal of Bone and Mineral Research, October 2008, 23(10:1552 1560, "Older Men With Low Serum Estradiol and High Serum SHBG Have an Increased Risk of Fractures"
8) Clinical Endocrinology, Sep 1995, 43(3):331 337, "Hypothalamic-pituitary-testicular function in end-stage non-alcoholic liver disease before and after liver transplantation"
9) The Journal of Clinical Endocrinology & Metabolism, May 1 1987, 64(5):1083-1085, "DIETARY LIPIDS : AN ADDITIONAL REGULATOR OF PLASMA LEVELS OF SEX HORMONE BINDING GLOBULIN"
10) Am J Clin Nutr, Dec 1996 64(6):850-855, "Effects of dietary fat and fiber on plasma and urine androgens and estrogens in men: a controlled feeding study"
11) Clin Endocrinol (Oxf), 2009 Dec 29, [Epub ahead of print], "Association of vitamin D status with serum androgen levels in men"
12) Cancer, 15 August 1988, 62(4):760 762, "Sex hormone imbalance in male alcoholic cirrhotic patients with and without hepatocellular carcinoma"
13) J Clin Endocrinol Metab, 2000 Jan, 85(1):293-6, "Diet and sex hormone-binding globulin"
14) Int J Androl, 1981 Dec, 4(6):639-45, "The effect of clomiphene citrate on sex hormone binding globulin in normospermic and oligozoospermic men"
15) Clinical Endocrinology, Oct 1998, 49(4):421 432, "The effect of six months treatment with a 100 mg daily dose of dehydroepiandrosterone (DHEA) on circulating sex steroids, body composition and muscle strength in age-advanced men and women"
16) Clin Endocrinol (Oxf), 1992 Feb, 36(2):187-92, "Serum levels of free and bound testosterone in hyperthyroidism."
18) Clin Endocrinology, Oct 1998, 49(4): 421-432, "The effect of six months treatment with a 100 mg daily dose of dehydroepiandrosterone (DHEA) on circulating sex steroids, body composition and muscle strength in age-advanced men and women"