Isn't it nice to have your dog shake your - or your friends' - hand when you ask him so? It's easier that you think...
How to do it
With your dog sitting, say the word 'Hand' (or 'Shake', or whatever you want to call this trick) and start touching your dog's left paw. As soon as he raises it and puts it in your hand, cheer him and - once in a while - give him a treat
Tips & Tricks
After a while, start saying 'Hand' without touching your dog's paw, and cheer him a lot when he raises his "hand". Oh, and don't forget to shake it!