Originally Posted by mihlbach

If your wheel stays aligned in the frame, then its probably not slipping. If you have your nuts cranked down properly, it takes some serious strength for them to slip. I'm a pretty strong guy (I could leg press more than anyone in my high shool except for one really fat guy who carried around his tremendous weight everyday), and I've never ever had an axle slip, no matter how hard torquing the cranks...not even on my low geared singlespeed mtb, mashing as hard as I possibly can up steep hills. WIth a higher gear typical of a street fixie, you can't exert nearly as much tension on the chain as with a MTB.

Your chain could be stretching...if you are lubing a dirty chain then you are washing grit into the inner workings of the chain and dramatically accelerating chain wear. I've noticed that if I get lazy and lube the chain without thouroughly degreasing it first, the chain will slacken noticably after 20-30 miles or so because of all the grit that I've washed into the chain. If I degrease my chain first, I can get more than a hundred miles without the chain getting too slack.
Also riding in wet or otherwise dirty/gritty conditions accelerates stretch no matter what you do. I get way way more miles out of the chains on my dry weather bikes than the chain on my rain beater.