It's a mental thing mate, don't know that any supps will help with that. You have to be switched on and focused before you start the workout...be looking forward to it. I think about what why my goals are for the workout, how many/which exercises I'm gonna do, what were the weights/reps I did last time, what will I do this time etc.
At the end of a set I'll log the details, think about how I went, what I'm gonna do next set. The whole workout I'm thinking only about the workout. That happens about 98% of the time. A few times my mind has wandered during a 12 rep set...thinking about work or whatever, but I force myself to snap out of it the next set.
Another thing to consider if you've got an iphone or ipod touch is to get the Gymbuddy app. You can set a timer in between sets. It'll start dinging after the time expires and wake you up out of your daydream.