Safety debate aside (there are ton's of great accredited websites out there where you can get the information to make an informed decision about things)
I have found there are a couple of do's and dont's for blowing a guy with a condom.
1. The condom--there are some flavored ones out there that are pretty good... kiss of mint, cola, strawberry... they are good.. but the flavor wears off too soo (or I suck for too long).. I'd suggest starting with a standard non-lubed condom. You may have to taste test a few to find something that works. Also to note on condoms... there are a TON of them out there... spirals, extra "head" room, ect.. that enhance the feelings for the wearer. These can be great and actually can produce and even better BJ experience.
From personal experience, one of the top 5 bj's of my life was with a condom on.
I would however stay away from anything pre-lubricated. 9 times out of 10... whatever lube they used tastes nasty!!
2. LUBE!---it's critcial! .. Again.. you need to taste test this one. Astroglide is my personal fave. I'd stay away from ANYTHING oil based. Put it on the guys dick BEFORE you put the condom on. This will provide some internal friction (try jacking off with a condom and lube for reference). Once the flavor wears off put a thin coat of lube on the covered cock and go back to town... You shouldn't have to reapply after that. Just don't let the latex or your mouth get dry.
3. Water... keep a glass on hand.. you'll need it to keep you mouth from getting all dry. Also try some uber cold ice water. Take a big sip.. hold it in your mouth for a second.. then swallow.. then back to sucking.. it's a really cool sensation.
4. The "Tip"--If you have a particularly sensitive gag reflex, make sure that the tip/head of the condom isn't just hanging there, rather stretched tight over the head. Be careful with this on... can gag you without warning.
As far as technique goes, be sure to start out slow and get used to the feeling/taste of latex.
It can be exremely hot for both the wearer and the one giving the BJ. Although it's not the real deal, it can often be hotter. You just have to get alittle creative.