If you can't deposit, then freerolls are great. The main issues are field sizes can be very large, payouts might be capped, and the structure might be great. Also, I think a freeroll should be worth your time. It doesn't make sense to play a freeroll for 8 hours, just to make $2 for 1st place.
If you can deposit, then you can be a bit more selective and play games that have reduced variance. For example, smaller fields will have less variance than large fields. As will bigger starting stacks, more starting big blinds, and longer blind limits will also have less variance. On the down side, they do last a lot longer compared to the turbo and hyper turbo mtts but, if you can play lengthy games, then it'll give you better chance to do well.
Finally, bankroll management is going to be key in building your bankroll. It's impossible to build a bankroll if you don't have one and loose/no bankroll management is how you lose it all.