How to give a really good blow job?

Put enough sucking pressure on it, look him in the eye when you suck. And go deep.

THOSE ARE GOLDEN RULES

Have you asked your guy for feedback? If he knows what works best on him, you can't do any better than that.

Generally speaking, pay attention to the base of the head, that ridge is very sensitive. Also, stimulate orally and play around with the testicles. As long as you stimulate the sensitive areas, most anything else you do in transition will be good/ok. Well of course. NO biting ;)

I like to start out slow and just lick up the under side. Do some teasing, and then put the head in my mouth and swirl my tongue around the head. I also love to look up and have eye contact at times. This is easiest if he is laying flat on his back. It's hot to see the pleasure you are causing. Always keep a hand on the base of the penis if you aren't deep throating it. Don't squeeze though. And don't be afraid to move your hand with your mouth. Or put the penis deeper in your mouth and put your hand on his balls.

Also, if his balls aren't too hairy, you can take one in your mouth at a time and just lightly suck on it and move your tongue on it WHILE still using your hand on his penis.

Main tips-
use your tongue a lot. Wiggle it on the under side, swirl it around the tip, etc.
have your lips around it, not open like in porn
don't forget the balls :)
eye contact
pay attention to his noises/actions
ENJOY DOING IT! It's soooo easy to tell if you are doing it for him OR doing it because you WANT TO. Have fun.
Swallow! The easiest way to do so is put it towards the back of your mouth and just start gulping when it starts.

OH and I forgot a very important part! Just because he is finished cumming, doesn't mean your job is done! I *LIGHTLY* suck on it until it gets soft. Not completely soft because he might just get hard again, then. Unless that's what you want ;) And I say LIGHTLY because I assume it is most likely very sensitive after orgasm.