Most of us would define erectile dysfunction as the inability to get an erection. Nothing is more embarrassing than not being able to "rise to the occasion." However, the real problem for many males is actually not so much getting an erection but rather keeping it. Consider this definition from the Cleveland Clinic for erectile dysfunction:
"The inability to develop and maintain an erection for satisfactory sexual intercourse or activity in the absence of an ejaculatory disorder such as premature ejaculation." 
Many men with erection problems find that they can achieve an erection (one way or the other) but lose it rapidly. This causes every bit the level of panic as not being able to get an erection in the first place. It also causes complete confusion. "How can I get an erection and then lose it so fast?"
Look at these desperate forum posts where the men involved express complete bewilderment and despair at the situation:
"I began noticing, in the last couple of years, that even with Cialis I couldn't always maintain an erection. In the beginning I noticed that fast, vigorous sex with my 20-something wife, would sometimes have a desensitizing effect on my penis." 
"For the past few years I have had difficulty maintaining Erections. It has gotten worse over the past year. I was taking mild doses of Viagra with some success and after time it did nothing. I would get hard, and then lose it." 
"Every morning I get up I can get an erection no problem and actually feel horny after the certain herbs I have been taking..I do think the standing up thing helps now I can maintain the erection with no limit on time with very light stimulation. But if I leave him he will lose partially fairly quickly..not immediate tho " 
"Then the next thing I noticed was ED. I was having some 'special time' with my lady, and I lost my erection. It was really crushing, but still I tried to forget it." 
I could list other examples from the forum, but you get the idea: this is a very common men's issue. So what causes a rapid loss of erection? Of course, there can be more than one reason, but we list some of the most common reasons below:
1. Venous Leakage. This refers to any vascular (blood vessel-related) condition that accelerates the loss of of blood from the penis during an erection. This creates a state where blood flows out of the penis as fast as it flows into the penis and is the equivalent of a boat taking on water more quickly it can be pumped out. Venous leakage can occur due to many causes, a subject we will discuss a bit more below. You can also read about it in my link on Venous Leakage.
However, the key concept is that as blood flows into the penis, there are veins that handle outflow that are supposed to get "pinched off." If this pinching off does not occur, then blood literally flows in and right back out.
NOTE: Always get check out by a doctor if you have erectile dyfunction as there are often medical conditions that underlie the condition.
2. Low Testosterone. So what is the leading cause of #1? It is something you might not guess: low testosterone. It turns out that testosterone is absolutely critical for the internal tissues of the penis. When the penis loses proper testosterone levels, nerves and connective tissue literally begin to "wither" and "atrophy". It is similar to a garden that loses its water supply. For details on what testosterone levels typically lead to venous leakage, see my book Low Testosterone by the Numbers.
3. Loss of Morning Erections. Anything that negatively affects morning erections can the chances for venous leak. As I outline in my link on The Basics of Morning Erections, this has to do with oxygenation. Morning erections give a proper oxygen and nutrient supply to the internals of the penis and, without them, the penis literally begins to die off internally.
4. Smooth Muscle Issues. Smooth muscle relaxation and tone is critical for for erectile maintenance and anything effecting this musculature can potentially lead to venous leakage. 
5. Low Nitric Oxide Levels. Now I don't have a study to prove this one, but anecdotally many men have noted that they lose erections quickly and, once they went on Viagra or Cialis (the classic PDE5 Inhibitors), the problem of losing their erections seemed to disappear. This definitely does not really fit in with definition of venous leakage or classic erection loss issues, but it does seem to work in some cases. In this case, efforts to improve endothelial function and restore more youthful nitric levels, something I dissuss at length on this site - see here - should help greatly.
1. Increasing Baseline Testosterone Levels. As mentioned above, the inside of your penis literally "rots" without enough testosterone. And so one strategy that may help many men that lose their erections too quickly is to simply raise baseline testosterone levels up to allow for self-healing. There are many lifestyle factors that I outline on my site, such as Sleep, Losing Weight Too Fast, Being Overweight, Depression, that, if corrected, can potentially lead to significant increases in baseline testosterone. Still other men will likely end up on Hormone Replacement Therapy or Clomid to raise their testosterone. (Talk to your doctor for advice on the latter of course.)
But the important thing to note is that a number of studies have shown that increasing testosterone (significantly) will lead to greatly improved venous leakage. Increasing testosterone will also boost nitric oxide levels and help with many other male health-related issues.
2. Increasing Morning Erections. There are many things that you can do to increase the number and hardness factor of your morning erections. Many great examples of things that can increase your morning erection counts are give on the Peak Testosterone Forum, including HRT of course.
3. Kegels. One thing that can really help some men are "doing Kegels." These exercises, often associated with female urinary incontinence, can actually really help men by restoring smooth muscle tone to the all-important pelvic floor muscles. See my link on Kegel Exercise for Men for more details.
4. Urological Visit for Venous Leakage. Venous leakage is the realm of urologists and is usually diagnosed by the same. There can be somewhat uncommon causes of venous leakage, such as Peyronie's Disease, that a urologist can help identify. Peyronie's Disease is scar tissue, or a kind of plaque, along the spongy "storage containers" (corpus cavernosum) within the penis. This scarring can make it where the penis can no longer traps blood as efficiently. But the important point is that one should get checked out by a doctor if you are experiencing an unusually rapid loss of erections.
Again, I would highly recomment discussing your situation with your urologist, as this is their "bread and butter". Again, there can be a variety of reasons that one can lose an erection quickly and a urologist can do the real testing that needs to occur. Furthermore, he can directly test for venous leakage using certain standard procedures, which were outline rather humorously by one of the Peak Testosterone Forum posters here.
7) http://www.gyneflex.com/kegel_article.html (A reproduction of the original article by Kegel).