I use graphite powder on my strat. It works, as in, it helps a tiny bit I suppose. And I use the whammy bar, and bend strings all the time. I have heard graphite is the best remedy from many guitarists. I've also heard to use oil. Jimi Hendrix used some kind of hair oil on his strings. But we all know how good he was at staying it tune.....

But from my experience it has a lot more to do with the age of the strings. After a week or two the strings start to go out of tune. Especially when you abuse them. While I'm on strings, when you put a new set on, it also helps to stretch them. String the guitar and get it in tune. Then pull each string away from the neck a few inches and hold it for 10 or 20 seconds. This helps to keep it in tune right after stringing the guitar. Like, if you are at a show, or you just don't like re-tuning for two days everytime you pick it up.

Another thought: It could just be your guitar. Fenders are notorious for going out of tune. But some are worse than others. Personally I wouldn't recommend a Fender. But I would recommend a GOOD Fender. I play one almost exclusively, and personally I think the tone is worth the sweat to keep it in tune.