Agree with Fish if it is a cash bonus.
For a Bonus Play, If it's a decent size Bonus Play, say $500, I pick a big dog and bet it with the FP. Then play the favourite at another book of mine. Ends up being a bet on a heavy favourite to win close to the amount of your Bonus Play.
Ex. Dog @ +200 - FP risking $0 to win $1000
Fav @ -220 - risking 1000 to win $454
Would like to hear if there are better ways of using a FP.