For me, I've had wet dreams (even lucid ones) and I have never abstained for more than three days.
What I'm writing below isn't exactly fact but is information based on my experiences.
This is when I have wet dreams the most. I abstain from touching myself at all for one day/two days. Before going off to sleep, I look at something arousing (not porn. No nudity allowed). The trick is to get your mind to fantasize rather than actually see anything. I've had wet dreams because of this before so I'm using this as an example.
After abstaining the whole day(or however long), while you are in bed, look at this photo. (it's Yvonne Strahavoski/Sarah Walker from Chuck). She's not really showing any skin. Now, while looking at the photo, think of her doing something to arouse you(like a really slow striptease). Don't think of anything above the level of PG-13. The objective is get an incredible hard-on, your heart racing, and your brain hard at work fantasizing.
Now, no matter how turned on you are, DO NOT touch yourself.
Then watch the video below. It's Yvonne in lingerie. Again keep watching it but do not do anything until you reach a point where you cannot control yourself. Then, run a finger from the bottom of the shaft all the way to the top. Do this once every two mins but do not masturbate or do anything.
While still in this state of incredible arousal, turn off everything and drift off to sleep while fantasizing about Yvonne.
Set an alarm two hours before the time you usually wake up.
If you still haven't had your wet dream by the time the alarm rings, then do this: turn off the alarm and manage to stay awake for at least two mins. Again, drift off to sleep fantasizing about Yvonne. You can touch yourself a little bit. There is a very good chance that you will dream about Yvonne and have a wet dream.
There is a higher chance of it happening in the morning during REM sleep. Try it. It always worked for me. The objective is to drift off to sleep in a state of incredible arousal.
VIDEO
http://www.youtube.com/watch?v=sLmMGeQkGQs