How do you come up with an art style?

I want to make another Space Invaders clone, but I'm having a real hard time coming up with an art style for it. How do you all go about it? Trial and error? Research? Sometimes I can just sit there and it happens, most of the time though everything I create looks plop. Any pointers?

Out of interest, has anyone hired an artist for their game? What kind of money can you expect to spend for a decent job? I've seen some advertised but I don't want to waste their time if it's too expensive. I know the only way to find out is to ask but I'd like a rough idea of what I'd be looking at.

Cheers.

If you are planning to make a really good game, it might be a good idea to get up a fun prototype, then put it on an artists website asking for someone to split profits. Of course, only do this if you are very serious about the game!

I did this and got multiple offers.

That's a great idea but I'm a pessimist so I'd be afraid of the game not selling and everyone hating me lol. It's something to consider though once I come up with a game I'm really happy with.