Steps for Mineshafter!
1. Register an account in http://minecraft.net .
2. Use the same email and register in http://mineshafter.appspot.com .
3. Link both of the accounts in http://mineshafter.appspot.com/settings .
4. Set up your username [the name you have on the server] and your skin & cloak.
5. Download the Mineshafter Client Proxy in http://mineshafter.appspot.com/downloads .
6. After you have done that, put it in a file and double click it.
7. !IMPORTANT! Everytime you would like to play Minecraft, you have to click on the downloaded file. The regular minecraft launcher shall be useless.
8. Login with your MINECRAFT account.
9. Play!
10. !IMPORTANT! When it asks to 'Update Now?' Click no.

PROBLEMS?
If you have a problem running the Mineshafter-proxy.jar [try double clicking] create a .bat file in the same file and write :
javaw.exe -jar Mineshafter-proxy.jar
That shall be your new Minecraft launcher. Use this only.