Talking to the guys either at AETC HQ, HAF HQ, or at the UPT bases, the number of folks who "wash out" of UPT remains pretty steady at the 15% - 20% level.
Of course, Initial Flight Screening will find the folks at first who won't be able to hack the pace at UPT, plus most students who are having problems grasping a certain phase will first wash back a class in order to get them to grasp the concept through more training.
So, if you make it through IFS, chances are about 80% you'll make it through UPT. You may of course take longer than originally programmed (but don't worry, there are many who do, for a variety of reasons)....
Best guarantee to make it through? Work your tail off, study hard, and when you think you given it enough, work / study some more....
Good Luck!