Being a winning slot machine player is to some extent impossible. All slot machines are specifically designed in order to give the house a long term edge, so the house will always come out ahead if you play long enough. The only real way to counteract the house edge on slot machine games is to play a game with a really big jackpot, bet the max every time you play, and hope that you hit the jackpot. Then when you do hit the really big jackpot, guess what you do next? Stop playing that game.
Don't get me wrong. I'm not saying that you shouldn't play slot machines. In fact, I think slot games, especially the really good ones, are a lot of fun. But you want to keep in the forefront of your mind that mathematically, what you're doing when you're playing a slot machine on a long term basis is paying for entertainment. You can calculate how much you're paying for that entertainment by multiplying the house edge times your average bet times your number of spins per hour.
For example, if you're playing a slot game with a payout of 95%, then the house edge is 5%. (The casino keeps 5% of every bet you make long term.) And if you're average bet is $3, then you're going to pay an average of 15 cents per spin to the house. (5% times $3.) Assuming you're making 500 spins per hour, that game costs you $75/hour to play, which may or may not be a reasonable price for you entertainment. That depends on your bankroll.
Something else to factor into your calculation is how much the perks and bonuses you're getting back from the casino are worth. If you're playing in a land-based casino where you're getting free drinks while you play, then you can subtract the cost of those drinks from you're hourly cost. (Or you can add the cost of those drinks to the value of the entertainment you're receiving--it's just a matter of perspective.) My recommendation is to drink top-shelf liquor and premium beers in order to maximize the entertainment value you're receiving. A Heineken can cost $4 a bottle in a nice restaurant. Drink two Heinekens an hour, and you've just lowered what it costs you to play each hour from $75 to $68.
Slot clubs also give back a percentage of your losses each hour, so definitely be sure you join the casino's slot club and ALWAYS use your card to track your play. There's absolutely no reason not to do this. Casinos also reward their larger slot players with comps like meals, show tickets, and free rooms, which all add up to reduce the amount of money you're spending each hour that you're playing on their machine.
So how to be a winning slot machine player? I'd sum it up by saying know how much it's costing you to play each spin and each hour, take advantage of all the comps and the perks, and go for the big progressive jackpot.