Play Very Very tight! Write down a list of hands you'll A) call with pre flop, B)raise with pre flop (and re-raise, really JUST AA and KK, V V rarely AK to get rid of the muck) and thats it. Dont play out of your range till the last quarter, then mix it up with the real players left.
Outlast the sit-outs. After the first hour the field will have at least halfed, half this again with the sit out's, they will drop after the break from the blinds.
Get VERY lucky. Most freerolls pay 1% of the field, 27 places in 2700 players on Full Tilt. You wont cash on a regular basis as a learner.
My advice is not to play freerolls to learn. You wont get a good feel for the game until there is cash on the table. Make a deposit and play $1 SnG's or similar. Everyone wants something for nothing!