What are rules for an overcall that have wide acceptance?
I do a lot of "weak jump overcalls".  That is to say, if my opponents open with 1H or 1S, and I have 6 clubs or 6 diamonds, and 6-9 HCP, I would invariably jump to 3C or 3D respectively.  (Well, I say invariably, but vulnerability and points below the line would actually be taken seriously into consideration.)

With 7 or 8 clubs I might jump to the 4 or 5 level.  I'm that serious about overcalls.

This is all based on the same principle as my favoured, weak, 1NT system.  It's better to get in there with a balanced hand and 12 HCP than it is to give the opponents all the bidding space they need.  Even though you will sometimes get severely doubled and go down in flames.  It's a risk you have to take, when quite often average players just have no idea to deal with aggressive overcalling, and you can totally take apart their game accordingly.

I'm always amazed when I play online with a randomly picked-up partner, and they chastise me for weak overcalls and general interferential play, when the alternative is to let the opponents bid an easy 1NT-3NT or something and take the match without any difficulty.  Going down and giving points to your opponents is not a crime against Bridge play - as long as you still have a chance of coming back and taking the match!  (I will agree, though, that keeping on overbidding and refusing to let the rubber end, when you are thousands of penalty points down already, is very irritating.  I assume the people who do this are mostly obnoxious kids...)