I'd let another guy jack me off (and have before), but I think most men would agree, you can do it better yourself. I find that the one position that works best for me, is if I'm sitting in his lap or laying back on him in bed. Maybe that way it is just more like he's wanking himself.
My pointers:
1) Definitely pay attention to what he says when giving him a handjob. If he says "tighter", "faster", "play with my balls", then do it.
2) A good handjob is about more than the cock. Use that other hand to explore his body. Tweak those nipples, tickle his balls, etc.
3) A finger can be a wonderful thing. Let me him suck it or get it wet and find his prostate (even if only externally).
4) A good position (at least for me) is laying in bed with him in front of me and leaning back on me. I use one hand to work his cock and the other I usually place with my thumb on the very base of his cock with my finger wrapping around underneath to play with his balls and massage his perineum.
5) If he's a shooter, don't aim for his face! Nothing sucks more than getting a great handjob and just as you cum getting spooge in your eye!