Some of the best information on playing the slots with an advantage was originally published in a book titled Robbing the One-Armed Bandits by Charles W. Lund.  If you order this book be sure to get the expanded edition.

People often ask if slot machines can be beaten.  In other words, can players achieve an advantage on some machines?  The answer is, “it depends”.  It depends on the type of the machine, of course, and whether the machine was designed to offer bonus levels and/or progressive prizes.  Some machines can be left in an advantageous state when a player leaves.  If you know what you are looking for, these machines can be taken advantage of.  Happy hunting!