Good luck

Well I am more of a let the mind wander guy, so I let the visualization come to me and I do this by relaxing. After some time I start experiencing (not vividly) impresssion of an experience.
Like I see a woman and hear weird things like "The bus isn't going to be purple in the weekend" o. O and I just observe this as it already was a dream or information of my subconscious.
So I have never tried to concentrate on a self made visualization for very long, well I have as a beginner but at those times I just fell asleep unaware so I didn't got the time..
I do however meditate for 40-60 min very often by concentrating on my breathing so I guess that can be seen as a concentration practise.

I haven't read about Sivsaion's simple visualization tasks so you have to describe them to me. ^^

The time of the WBTB is important because the longer you stay up the more deprived you get of REM, so the brain sort of force itself to REM.
This can also be achieved by not sleeping for 24 hours and then go to sleep, I wouldn't recommend it though, the WBTB is a healthier option.

What I am currently doing is to go to set an alarm to wake me up 1 hour later but not more than 2 hours, and make sure that I get atleast one hour of sleep.
Then I stay up for atleast one hour but still not more than two hours and actually study (Some research shows that sleeping after studying helps you remember better, so I take advantage of that here). Then I go to bed and start visualizing and the idea here is that as soon as I start relaxing my brain will perform a REM-Rebound and then I just do what I described before.

This is a new kind of WBTB variation for me, and I usually don't even WBTB so it takes some time and patience to get used to staying up when all you want to do is to sleep. xD
And yeah to study on top of that is a really good concentration exercise!

Last night I only stayed up for 30 minutes, but tonight I am going to stay up for the full hour because that is apparantly necessary in order to trigger the REM-Rebound.

More info about REM-Rebound can be found here: REM rebound - Wikipedia, the free encyclopedia

So yeah if you are interested you can read about my attempts in my Dream Journal and if it provides me regular success you might consider trying it out.