That said, there are some techniques for winning at slots, or even having a go risk free. I'm writing this having never had a decent slots win myself (decent being measured as at least 100x stake as per the rules of the winning screenshot forum here!), but as you will have seen in some of my blog posts, I've certainly given it a try.
Now slots play is not part of Matched Betting, as there is normally risk involved. There are two important factors in whether to attempt a casino or slots bonus; is it risk free, and or is it expected value positive (EV+).
A risk free casino bonus is as it suggests. Play through the amount as per the terms and conditions, normally on a specific game, and if you don't win, you'll get your stake back. These are no brainers and a licence to print money. Sure you're not going to win all of them, but you'll be able to double up on a few of them and take you money out which is great, and if you lose you'll get the money back anyway. It's a shot to nothing that's worth taking all day everyday (which is why they aren't as frequent as they used to be, but they are still out there are William Hill in particular).
Expected Value calculations are slightly different and don't remove the risk from the casino game. What they do is make it more likely that you'll get out in profit. Expected value is based on the wagering requirements for the bonus, and the expected return from the game you're playing. It's a subject that needs more words that I've got here so until I write a full page on it I'll refer you to this excellent piece on expected value at Free Mobile Casinos.
To close, let's look at some of the slot wins people have had off low risk or risk free bonuses and that let me dare to dream! They're certainly possible, but the same warnings go with matched betting..........don't gamble!