My advice is that you never stay at any one machine longer than the time it takes the machine to eat:

5 if you are playing nickels.
10 if you are playing quarters.
20 if you are playing dollars.

Think about it. 5 worth of nickels is 100 of them. If you're playing a machine which requires 3 coins, you can try it 33 times before moving on. And that's if you have no winners whatsoever. But don't be surprised if you can't locate a 2 or 3 coin nickel machine. And if you want to play those 45 coin video specials, we'll get to you, later.

10 worth of quarters is 40 of them. At a 2-coin machine that's 20 pulls or pushes. If 20 attempts don't get you going, move on.

20 in dollar tokens provides 10 tries at a 2-coin machine. Not many, but if you follow my advice you'll find out it's plenty. Dollars normally pay back at a higher percentage than quarters or nickels. You should see winning combinations sooner and with more regularity.
Whether nickels, quarters, dollars or higher, stay at any machine which allows you to do so. You're going to leave if you lose your 5, 10, or 20. If you're not losing, stay.

As soon as you reach your first winning goal, Dead Bolt half of it. Keep playing. If you reach your secondary goal, then either remain at the machine which is paying-off or move up to the next level. However, you must define the next level for yourself. Will you move up from quarters to dollars? Or does moving up mean remaining at quarters and initiating an attack on the Progressives?

If you are losing, continue to move from machine to machine until either of two events occur.

1) You connect will a machine which allows you to play. In which case you continue to play until or unless the machine changes its mind and drives you away.
2) You lose half of your stake. In which case you drop down one level. Dollars drop to quarters. Quarters drop to nickels. Nickels continue to play until 90% of the stake is gone (never let'em take all the money). If you are forced to drop, begin anew. For example. You start with 100 as your stake. You play dollars. You lose 50. You drop to quarters. You start fresh. You begin with a "new"stake of 50 and continue as before. Ditto for quarters. If you continue to lose, quarters drop to nickels and continue playing until 90% of the original stake is gone.

Obviously, if you start with dollars and need to drop to nickels you will be at least mildly depressed. But you'll still be playing and there is always the chance for a turnaround.
Sound too conservative for your tastes? Too bad. If anything, my plan might be too aggressive. For example, I did not suggest that you always leave a machine which has recouped 50% of what it has given out. What if you stick in your 20, and run it up to 100? If you were conservative, you would keep playing that machine unless the credits decreased to, say, 60, then leave. The aggressive player might keep playing until the machine had reduced them all the way down to the original 20.

If your original stake for this particular session is 100, that's 2,000 nickels! It is also 400 quarters. With 400 quarters you can "test" 10 different machines. If you do so and can't find a way to win, you must be prepared to acknowledge that it just might not be your day. Or, if testing 10 different machines doesn't sound like enough, then start with a larger stake. Ditto for dollars.

Another informal survey result. What did all Slot players feel was the most common mistake they made? They all agreed, nearly 90% of them, winners and losers, that the most common mistake was staying at a machine too long. It was not knowing when to quit. If you follow the plan, you'll know. The hard part is being able to exercise enough self control to actually do it.