Although I won't claim to be a WILD expert, I have had a good few. But I used to have the same problem that you're talking about. I used to force myself to stay focused and when my mind wandered I would will myself to keep alert. But for me at least I have to let my mind wander just enough or I couldn't stay relaxed for too long. You have to kind of fall asleep but keep just enough consciousness to keep your goal in the back of your mind. (I know, weird to explain.) At first it's kind of hard to find a good balance of alertness and being half-asleep and I can't explain really what it's like but after a few times you'll know. Like in Mzzkc's Why You Fail at WILDs your main focus should be on FALLING ASLEEP as counterintuitive as it sounds, it's true.

One more thing, if you really need something to keep focused on you could try bengron's Sink Under Bed Technique (SUB). Helped me out a lot.