First, you have to know your slots...by playing them. I have a few at SY that I play with a little cash to watch for when they are primed.
Second, most FreePlay won't let you play anything with a ProgressiveJackpot -- so don't go hoping for a Wheel Of Fortume MegaMillions winner off $20free.
Third, never put more on a machine than you want to spin. You can add more, but removing it can be impossible (SY) or confusing (Harrahs). I go for about 10 spins of all lines at whatever rate I want 1-3 coins/line ($2-5).
Fourth, know that you can strip off your winnings and the FreePlay will stay. So, either talley in your head the winnings or cashout after a larger payout. You will end up with an "odd spin" of 3 lines at 1/per or something like that if you are playing 9lines. It's just that way.
Lastly, look for machines with interesting bonus games/free spins. That's where you will make $10 turn into $30 or more to take back to the tables or poker room (like I did on Sunday).