Well, you can try copying another game, that's coming out and hope that the non-serious gamer (or mother) won't be able to tell the difference. This happens all the time with poorly-made rushed movies that coincide with upcoming blockbusters, so why not make it work for you?
You can go with the old standbys, Tits and Gore. Also popular are big burly men, and dusty, grainy action images of soldiers. But of course this relies on you making the sort of game that needs those things on the cover, pandering to the widest possible audience. (obligatory call to stop making WWII games)
Why not try some subtlety? You could make your cover look like a book from the 1960's. Gamers seem to respond well enough to that to make their own, even. How about classing it up? Make your cover show that you are the Criterion Collection of games. Show that you love the product as much as you hope a potential buyer will. Or have some fun.
This approach is somewhat backwards. Gamers have to like the game to make to make alternate or fake covers, and that means you have to sell it first. All the games in the posts above were bought and loved with whatever cover they originally shipped with. I know that the quality of a game has nothing to do with its cover. And plenty of terrible games have decent covers, too. If that helps them sell, so be it. But your game is good, and so you're back to the original problem.
Well, I'm inclined to think that for every person who buys a game based on cover and back cover description alone, there's at least one who buys a game they heard was good, or read a review of and has already decided to purchase when they walk into the store. This wasn't always the case, but with the internet and all, fewer and fewer people are dropping $60 to find out if the game matches the cover quality. People buy used games on impulse but you don't make any money off that, do you?
So let's do this. Make your game good. That way word of mouth and reviews will do the majority of your marketing for you. Make your covers compelling. Not tits-in-your-face, balls-to-the-wall, hoo-rah exciting. And I, for one, will promise to at least look at your game in the store, which is more than I can say for the cookie cutter covers I see in there all the time. (See how nicely I avoided even mentioning the better-than-average cover of Rock Band?)
Or don't. Covers might not even be around in a year or three, thanks to digital distribution. But please, do find some other way to show you've put care and love into your game. We'll say thanks with money.
Thanks to Mike Mayday, who can be reached via wp.pl for the drawing I used in my Neverhood box art.
Fun fact: Spectre VR was packaged with a free copy of Neil Stephenson's Snow Crash way back when. How's that for cross-promotion?