There are a number of ways of doing that, some casino bonuses are quite simply EV+ (if you don't know what that is, then I've done a page on Casino Bonuses) in that you would expect to come out with more than you started with simply because of the games you play and the expected returns on them.
Some Casino Bonuses, risk free ones, are just no brainers. If you're sensible then you will either win, or get your stake back with no risk to the rest of your funds. You're not going to win every time, but you'll make some decent money in the long run as at some point you'll hit a decent win and every offer is basically a free shot at it.
Finally there are cashback offers. This is where you receive back a percentage of your losses after playing on a particular game. William Hill are running one of these today where you will receive back 30% of your losses. The best way to profit is by spreading out the maximum stake on one spin and crossing your fingers. The odds you are getting (given the refund of 30%) are much better than you'd normally receive. That doesn't mean you'll win, in fact variance means you could lose dozens of them on the spin, but when the odds are in your favour, you should be taking the offer all day long.