BUT I don't believe in the biblical idea of God as the great vending machine in the sky.
Gee, thanks for bursting my bubble Sirona......I was about to ask God for a Hershey bar.
Anyway, though I hardly pray, I'll take a stab at this topic. I'd imagine that, since prayer usually involves asking God for some type of intervention on some level (not counting prayers that simply give thanks), we are in effect suspending our own free will in the matter because we've turned the matter over to him, to his providence one might say.
I guess it might be likened to having a problem at work and going to the boss about it. Once we do, the boss's opinion now becomes a factor and usually then supersedes our own. We've handed the matter over, and along with it, some if not all of the choices on a possible course of action.
When a jw or most other Christians for that matter give their lives over to the service of God, the expression is often used that they are doing his will and that they are letting him now decide what's best for them. It's their choice to do that, which is an act of free will in itself (putting aside for now whether most jws are in fact pressured into making such a choice), but one in which they seem to be invoking God's will on their future life course and decision-making, hence surrendering to a degree their own free will in favor of God's will.