Once a machine pays off, don't play it again because it won't pay off again for some time.
The machines have absolutely no ability to keep track of what has happened in the past. The results are totally random, the slot machine is as likely to pay off on the next pull as it would have if a jackpot had not been won.

Slot machines near the entrances to casinos pay off more than other machines.
This belief is based on the assumption that winning machines near the entrance will attract passerby's into the casino. However, today there is no need for casino operators to attract people to play slots, so if it was true in the past it most likely doesn't apply to today's reality.

Casinos can change the odds of a machine so beware of any casino employee fiddling around near the machines.
The payoffs on every slot machine are coded on a computer chip inserted into the machine when it's manufactured. Casinos, slot technicians, etc. cannot alter the payoffs.

If a slot machine advertises a 95% payback then I should expect a return of $95 for every $100 I put into the machine.
The payback percentages are long term percentages. The long term outlook allows the casinos to average income from the machines over time, which is much longer than the time you would take to play $100 in a machine. In the short term (the time you would take to play $100) the machine could pay off a lot more or a lot less than the advertised amount.

You should play one coin at a time to get more chances at hitting the jackpot.
The chances of hitting a jackpot are small. In most cases, playing the maximum number of coins increases the payout rate percentage, which is actually your better bet to walking away as a winner.