To the above advice I would say if you see a FP come up for a ride you want, take it no matter the time, then try to modify it. It seems to be easier to modify a time that to get the time you want right off the bat--plus, once you have it, the times for that attraction will display at the top of the screen rather than possibly having to scroll each time you refresh.
We actually had to split into single riders sometimes to get the extra FPs we wanted, and then modify until we all overlapped/had an early enough start time. We weren't trying to get any headliners, either--I know that can happen, but we didn't see much availability (Mardi gras week, so pretty busy but probably lighter than Easter).