At essence, Online Slots are games of chance, with no true strategy. There are a number of different ways to select your bet, and the amount you are going to bet; but other than that the game is left to luck due to the use of random numbers making it impossible to predict the next spin.
The biggest mistake made by slots players is to be unfamiliar with the game they are playing. Set aside some money from your bankroll that you can use to familiarize yourself with some of the slot variations that you would like to try. Learn the button layout, the pay table and the multitude of betting options. And even better, if an online casino has offered you bonus or free money as an introductory offer, use this money to find and familiarize yourself with a favorite casino games.
Closely related to this last point, be aware of extra features and gambling options that are only available depending on the amount you wager. Again, know how the money works! Then, you can choose a slot game that fits your budget. It is easy to go through many spins an hour paying Online Slots and thus it is easy to burn through your bankroll in perhaps less than an hour. So decide how long you wish to play and choose a slot machine that has a wagering level that will allow you to meet your time criteria. You cannot control the outcome of the spin, but you can control how long you get to play for and the amount of money you spend.
Every player will find it useful to set a maximum and minimum win/loss level that must be adhered to. As with any game of chance this is especially important to Online Slots. Playing responsibly will do more for you in the long run than going for it all in one session.