The best strategy for slots - play the slot machine with the best odds. See the average payouts of different online slot machines: http://www.slots-online.ws/slots-payouts.htm
At the bottom of the page you'll find the slot games with relatively high build-in payoff.

Another tip for progressive slots players - play the slots when the jackpot is high enough, at least 5 or 10 times the minimum value. See ideas at the end of the article: http://www.winnerstrategy.com/system.htm

When you play progressive slots you should BET MAX to be eligible for the jackpot win. So find the progressive games with the bets you are comfortable with.