to something like Bosto's favorite:
To change you skin,
1) find the skin you would like from one of the many Minecraft Skin sites such as http://www.minecraftskins.com/
2) download the skin .png file to your computer and save it somewhere that you can easily find it
3) in your web browser, go to http://minecraft.net and log in and click on the Profile link
5) on the Profile Page, go to the Change Your Skin section. Ignore the "Start by Downloading your reference skin and editting it" part and just go straight to following:
Click on Browse... and when the file selector dialog opens, select the .png file you already downloaded to your machine from a Minecraft Skin site
6) The image file name will now appear in the text box next to the Browse... button. Now to activate it, click on the "Upload image" button. Your new skin is now in place.
7) if you are using the downloaded version of minecraft (as opposed to a web client), close it and open it again. Go to Inventory and you will see what you look like with your new skin! Note that you can also look at yourself in third person by pressing F5 but unfortunately you can only see your back and sides!
