And while slot machines used to be programmed so that the blank stops above and below the jackpot symbol would correspond to more virtual stops than other symbols (so that near jackpot misses would often show up and trick the player into believing they are getting closed to winning the jackpot), this is no longer allowed in well regulated gaming jurisdictions like Las Vegas.

As for a slot machine's RNG, which ultimately determines the payout percentage, it is tested by gaming regulators to insure it is paying back a minimum percentage of funds that are deposited into it.  A machine with a payout percentage of 95 would be paying out 95 cents for every dollar put into it by bettors.  Even with only a 5% profit, it is easy to see why the casino never loses.

In highly competitive gambling markets like Las Vegas and Atlantic City, the payback percentages are often going to range from 90% to 97%.  Since there is greater competition, not to mention strict regulation laws that require a minimum payback for each machine, players have good chances of finding looser slot machines than normal.

Since each machine is regulated by a computer chip (which the odds are programmed into) the only way (in most cases) for a casino to change the odds of a machine would be to change out the chip, which contrary to some slot myths, does not happen in strictly regulated areas.  In other words, the "tightening" of slot machines simply does not take place.

Likewise, slot machines do not get "loosened" the more one plays them either.  Because of the irregular sequences of numbers created by the RNG - which are as equally random every time the reels are spinned - a player always has the same chances of winning every time they play.  It is not that a machine is "ready to pay" at any particular time, more so than it is the luck and timing of spinning the reels at the exact millisecond the right random number sequence is generated.

References:  HowStuffWorks.com & Wikipedia