First let's mention what doesn't work. The number of credits cashed out by the previous player is irrelevant. Likewise, checking to see whether the last hand was a winner is also irrelevant. The Random Number Generator in each machine is not affected by whether a machine is ahead or behind today, this week, or this year. This doesn't happen very often, but being alert to it can pay off.
Some machines are upright and some machines are slant tops.
Everything else equal, you can play longer on a slant top because supporting your own arms playing an upright machine can be tiring. Sometimes the glare off of slant tops is more than the glare off of uprights. Being next to or away from other players can be an important factor. Smokers may irritate you. You may see somebody you know and thus want to sit next to them.