What doesn't kill you is supposed to make you stronger.
It's an idea that speaks to basic human development, surviving by adapting and growing. With the advent of time travel technology, assuming the allowance of being able to move at least within the span of one's own lifetime and to coexist with different versions of oneself without doing irreparable damage to the continuum, the temptation to visit one's earlier self will no doubt manifest for some people. This will naturally lead to chaos for the more thoughtless among us who fail to think actions through to their possible consequences. These would be the same people who make the poorly conceived wishes that come back to bite them on the ass every time they run across a genie with some free time and magic burning a hole in the old diaphanous pants.
Though it may go unheeded, I'm going to offer a bit of advice to these people: just watch. That's right, just keep your distance and watch. Better still, don't go anywhere near your younger self or any of your ancestors. Marty McFly managed to come out a lot better than Ashton Kutcher, but considering that he nearly wiped himself and his siblings out of existence and narrowly avoided a disturbing oedipal incident as one of the many hoop jumps required to fix things, just save yourself the trauma. If you can't manage to keep your distance, consider writing a thoughtful note. Again, don't try changing huge things. Think it through.
If you're feeling Hemingwayish (it is too a word), you might pick out a slow day from your past and go kick your own ass. Sounds a little odd right off, I know, but think of the result. You'll toughen up your present/future self. Go back and do it again, allowing for some recovery from the first beating (maybe a few months to a year), and older you will reap the benefit of becoming even tougher. All the motivational posters say the only person you need to beat is the person you were yesterday. This would be your chance to prove them right.
For the love of you, though, don't get carried away or you may break your own spirit. And certainly don't do any permanent damage. And as long as I'm saying these things, don't kill your younger self. That's a dead-end time-loop paradox you really don't want to get into.
You know what? Maybe you're just not ready for a time machine. One of you is liable to mess things up enough that we start getting...corrupt politicians in government or something.
Wow, there's a thought. Forget I mentioned it.
Go contact the FCC. Tell them to reinstate net neutrality.