The RNG
Modern slot machines are computerized. So the odds are whatever they are programmed to be. The reels and lever are present for historical and entertainment reasons only. A Random Number Generator determines the positions of the reels. There is a Random Number Generator in the machine's software.
The rate of generating random numbers by the RNG is extremely high. When a player presses the Play button the most recent random number determines the result. This means that the result varies depending on exactly when the game is played. The result would be different a fraction of a second earlier or later.
Some professional gamblers observe that the RNG does not actually generate random numbers. The sequence of numbers is repeated by most of the RNGs. This behavior is due to poor programming. It is relatively easy to build pseudo RNGs with very long periods. No computer can complete a single period in the expected lifetime of the universe. It was Ronald Dale Harris, a former slot machine programmer, who knew the pseudo RNG code and seed values. It was he who discovered equations for specific gambling games like Keno. The fact that the next set of selected numbers would be based on the previous games played was revealed. However, this is impossible for most machines, because the RNG picks numbers even when the machine is not being played. So the numbers can't be exactly found in a sequence.

Percentage Of The Payout
Slot machines usually pay out as winnings 82–98 per cent of the money that is wagered by players. This is known as the theoretical payout percentage. The minimum theoretical payout percentage is controlled by jurisdictions. Law or regulation are used to establish it. A certain winning pattern is determined by every casino individually. It concerns the amounts they pay and the frequencies of these pay-outs. The main criterion for selecting the winning patterns on slot machines is that they should yield a certain fraction of the money played to the house. The rest of the money goes to the players.
A slot machine's theoretical payout percentage is set at the factory when the software is written. Changing the payout percentage after a slot machine has been placed on the gaming floor requires a physical swap of the software. As it is a time-consuming process it is done infrequently. There is sometimes a tamper-evident seal on the EPROM. It can only be changed in the presence of Gaming Control Board officials. According to other jurisdictions slot machines are randomly audited to ensure that they contain only approved software.
The Nevada Gaming Commission is working now with Las Vegas casinos on technology that would allow the casino's slot manager to change the game, the odds, and the payouts remotely. Only after the selected machine has been idle for at least four minutes can the change be done. After making the change the machine is locked to new players for four minutes, it displays an on-screen message informing potential players that a change is being made.

Machines Linking
Offering of a particularly large prize, or jackpot becomes possible when a group of machines is linked in a special way. A small amount is contributed by each machine in the group to this progressive jackpot which is given to a player who gets a specific combination of symbols. The amount of the progressive jackpot is far higher than any single slot machine's.
In some cases multiple machines are linked across multiple casinos. In these cases the manufacturer owning these machines is responsible for paying the jackpot.

Near-miss Programming
The reel display of modern slot machines is under computer software's control. It is possible to make the slot machine frequently display combinations that are close to winning combinations.
This practice is called near-miss programming.
There is a related phenomenon that is also sometimes called near-miss. Winning percentages programmed into the slot machine control the chance of a winning combination appearing on a pay line. The combinations above and below the payline are all roughly equally randomly distributed. This means it is much more likely that a winning combination will appear above or below a pay line than on the pay line. The occurance of this phenomenon is possible when abbreviated physical reels are used to display a win pattern based upon the RNG. In video slot machines the symbols that appear around the winning line are usually an accurate depiction of how the reels were mathematically modeled.
The issue of a near-miss above or below the pay line was also the subject of the Nevada Gaming Commission investigation. So long as the near-miss above or below the pay line was not specially programmed it was recognized legal. In other words, any other combination must be just as likely to occur as the near-miss. It is impossible to program the machine to show winning combinations more frequently than other combinations above or below the pay line.
In Australia near-miss programming, where a near miss is inaccurately displayed is not allowed either. Stop motion cameras are used by regulators for manufacturer's practices audit.

Slots Fraud
Sometimes mechanical slot machines and their coin acceptors seem to belong to cheating devices and other scams.
Coin acceptors have become obsolete in favor of bill acceptors and modern slot machines are controlled by EPROM computer chips. It is difficult to defraud these machines because they their bill acceptors are designed with advanced anti-cheating and anti-counterfeiting measures. Nowadays microwaves are used to defraud slot machines.

Know what you are playing.
Only the correct amount of money should be inserted into a machine. Your credits should be cashed out before you leave the machine.
Only if you have fun your gambling won't be stressful. If you feel lucky increase your bets and vice-versa.