And if you're just an occasional sports bettor and you want to lock in the profit rather than take the most +EV bet you can, hedge at a second book with the other side. So for example, you can bet +300 at a book giving you a $1,000 risk-free bet and $2,592 at a second book on the same game at -350. If the underdog wins, you're up $408 and you're done.
Now if the favorite wins, you lost $260 but still have a free bet left. So just for the sake of this example, let's say you find a second game where you can bet +300/-350. Do the same bet, except this time put $2,333 on the favorite and your $1,000 free bet on the underdog. Now you either win $3,000 on your free bet minus the $2,333 hedge for a profit of $666 or you win $666 straight up on the $2,333. Combine that with your loss of $260 in the first leg of the bet and you're up $406, roughly the same as if your free bet had won originally.
The numbers change depending on how good you are at line shopping and whether or not you have enough cash available to fully hedge a longer shot bet. But if you've got a few thousand bucks sitting around in savings that you don't want to put at risk, this is about as risk-free a way to get a substantial return on it for virtually zero work that you can find.
I made a spreadsheet to do all the math for me when I was grinding all the new user bonuses recently. If you want to use it, just make your own copy of it.
https://docs.google.com/spreadsheets...it?usp=sharing