The Parlay System is a positive progression betting system, whereby you effectively let your bet ride. If it wins, say on roulette, you leave your bet there for the next spin.
It is almost the opposite to the Martingale System, in that the punter is not interested in covering losses, only in chasing big wins.
The Parlay System is one of the lowest risk systems because it doesn't risk the gambler to losing a large amount money if they have a string of bad luck. The key question with the Parlay System is how long to let your bets ride... This is really up to you. Try varying it and see how you do; online casinos are a great way to test this system out.
The Paroli System
If you want a little more excitement, try the Paroli System which is souped-up version of the Parlay System. Rather than just leaving the original bet ride, the Paroli System involves also adding the original bet stake on top of that. In effect, it is the equivalent of the Martingale II system, but in reverse.