This is my experience/opinion:
There are a few factors. First, you have to be open to the experience and let it happen, not force it. Second you have to understand WHAT the triggers are when an anal orgasm is approaching. Lastly, you have to be a little uninhibited by the experience.
A man's anus is usually tight and the rectum is a small canal. When the penis penetrates through the anus into the canal it most likely will rub against the prostate, which begins to trigger a reaction in both areas. As the bottom you can squeeze your anus to tighten around your lover's shaft (which will drive him crazy) and this will lead to additional sensations for you in your rectum, which will begin to "lubricate". Huh? Lubricate?! Yes. Think about when you have a bm: if your rectum was dry it would make it impossible to go to the bathroom, except for loose stool (sorry to be graphic), so your rectum creates an environment to make it easier for the stool to pass through. When the rectum senses your lover's penis inside you the same reaction happens, but unlike using the toilet - usually over fairly quickly, shutting off the need for lubrication - the repeated entry into the rectum causes increased need for more lubrication in the rectum. The addition of prostate stimulation is an added bonus.
To point #2: You start to feel light pressure in your rectum and your anus will become very sensitive. Make sure to breathe, pant and - if so inclined - to moan. Your lover's pace will also help trigger a faster reaction. The stimulation will continue until you feel the need to "bear down", to push and you will feel your rectum contract and your anus pulse. Your lover should continue to penetrate you through the orgasm. As the orgasm subsides, the throbbing ebbs, he should slow down to let you catch your breath.
To point #3: If done correctly the above may, but most likely will, result in wetness inside your rectum and along your lover's shaft. In some gay porn films you can actually see fluid running down the leg of the bottom who is having anal orgasms, which often times is a combination of the top's pre-cum and the bottoms orgasm. So it can be messy. Start out using only a minimal amount of lube. Saliva is best as it is more natural than a silicone lube.
Can you have multiple anal orgasms? Yes. Can you have anal orgasms without stimulating the penis? Yes, but after you've had several anal orgasms in a session the penile orgasm is the icing on the cake. Hope this helps.
