http://www.PlayGuitar.com Here's a trick you can use for changing keys in a song. Basically you go to the V chord in the original key, and then you can shift to the I of the next key you're moving to. If you look close, the V of the old key is now the IV in the new key. You can use this tip for changing keys in a song nearly anywhere!
For more information, visit http://www.PlayGuitar.com/theory to learn more about the chord positions and chord theory on the guitar.