I assume you're talking about changeable clothes.
Basically, you have two options. Both will probably require that you set the cloth vertecies to a different material than the main body. One method involves replacing one texture with another, similar texture. This is useful when you only want to make minor changes to the look of your clothes, not massive shape changes. I hear you can get away with alot via bump mapping, as well. IIRC, Ocarina of Time used something to this effect.
The other option involves creating a "general" main object for default clothes, and an extra model for each type of clothes, with joint vertecies in the same position so you don't get seams. This is alot more work, but gives you more flexibility as you can change the entire model, not just the texture.
A system using both techniques would be a good idea. If you're using milkshape, I think I can help you with either technique, but I'm still learning myself, so I dunno how much help I can be.
Hope I could help!
~Underhill
I pity the fool who thinks 1337 is cool!