Check out this web page for details of how they actually work:
http://money.howstuffworks.com/slot-machine.htm
Generally the best way to win money is through the feature which you achieve by either getting 3 symbols or getting numbers totalling 8 (usually). Then this is where each machine differs, some of the features that are obtainable (money belt, step to nearest win, etc.) are common to a lot of machines and this is where knowledge helps. Knowing what feature pays out best on each machine.
Also in the Uk machines have to pay out a minimum (about 78%) by law, and generally you can judge whether a machine is likely to pay out. If it keeps taking you in to the features repeatedly, or you've just won a few quid and it takes you straight back in, then these are signs that the machine may be ready to pay out!
With the Jackpot prices going up in pubs (and the stakes staying the same), the jackpot pays out less often, making them harder to play/win!