If you have played slots online chances are you have played a Microgamming slot machine. 

Based on the Isle of Man, Microgamming is one of the best casino software companies, with the likes of Realtime Gaming and PlayTech, although Microgamming started it all with their first online casino in 1994. And their Viper Software is actually the most downloaded software on the internet, to date.  Providing software to over 100 different online casinos, Microgamming is responsible for the progressive slot machine, virtual blackjack table, and the multi-player poker table.

Now if you've picked a Microgamming Online Casino you will be playing at some of the most reputable slots on the net. Some other things to keep in mind when picking your game is the payout percentage and hit frequency.  These should be in the upper 90s.

Another thing to do is play the slots for a while. Any seasoned slot player will tell you that you have to get a feel for the game to start winning big.

Make sure your game has built in bonus rounds; these generate a much bigger jackpot.

Also, it's always a good idea to max bet. Only making the max possible bet will allow you to win the maximum jackpot.

Avoid, at all costs, the "high roller" slots, unless you are in fact a high roller. These slots can leave a huge hole in your pocket if not played correctly.

Other than that there are tons of other tops that I found on Wikipedia to help you with your winnings when it comes to online slot machines.