Well first, you need to be skilled enough to help someone else out. If you are confident that you are, then the above advice is great. But if you can't keep the guys plane in one piece for him for his first several flights, you won't be doing him much of a favor!

Personally, I can fly pretty well, but I don't feel I have enough skill yet to confidently help someone else out on the box (on planes, anyway... helis are another story). If there was absolutely no one else in the area, fine... but I feel the best favor I can do for someone asking me to help them out is to just introduce them to one of the very experienced guys at our club that trains new pilots.