1. Don't bet anything where the bet can push. MLB is actually not the best in this regard as BOL for example will cancel your bet and take away your freeplay if there's a pitching change. Stupid, and if you argue they'll eventually return it but you're in for a 3-4 day battle.
2. You want long odds. Parlays give you long odds if the book allows them, so parlay anything that can't push. Otherwise, the biggest dogs you can find within reason.
Google 'freeplay calculator'.
There is an exact cash equivalent for freeplays, anywhere from 50-70% depending on how you bet it. Ignore idiots who turn down freeplays because they are not cash bonuses. They are.