I hate it when mine does that! messes up my ollie and my Pop Shuvit fur shur! I gotta bail. Actually I'm a true Poseur.
If your skateboard is constantly turning or leaning towards the left or the right, there are a couple of possible causes. The most likely ones are either the bushings in your trucks, or your hardware.
Bushings are the hard, rounded plastic pieces in your trucks that squash down in the direction you are turning. They give varying levels of flexibility (usually measured in hardness such as - hard, medium or soft) to the turning capability of your trucks.
Your bushings could be a bit squashed on one side, this can happen when you are turning in one direction a lot. If this is the case, try and lean on the other side a bit, even if you aren't actually skating. This should even them up a bit. This tends to happen when a set of bushings are still fairly new. It can last for a couple of days up to a couple of weeks, it just depends how much you are skating.
Another possibility is that your bushings could be damaged. If there are cracks in them, it's a good sign that this could be the cause. You will probably need to replace them if this is the case.
The hardware I am referring to here is the set of bolts that hold your trucks to the board and your kingpin which runs through your truck and bushing.
It may simply be a case of your kingpin being a bit too loose or not quite set straight, in this case you will either need to tighten it up, or straighten it up.
It is also possible that some of the bolts may not be done up tight enough and the truck is twisting slightly. You should just need to tighten up the loose bolts. Most of the time it will sit straight simply by threading the bolts through the holes in the board and the truck, but sometimes there is a bit of wiggle room that can put them slightly out if the bolts are a little loose.