Great advice and tips in the previous posts. I agree with communication and enthusiasm being two of the most important aspects of giving a great blow job.
Here are two more minor tips to help when giving a blow job:
For the fellas, watch the facial hair. I love slapping another man's penis all over my face while I am sucking his dick, but have been told I have very scratchy and rough facial hair. I usually find it best to shave before I give someone head, as to not scratch or rub their penis the wrong way. When giving head, be mindful of how your facial hair might feel against their penis or scrotum.
For both women and men, be mindful of your fingernails. Similar to facial hair, no one wants their penis scrapped or scratched while receiving head. Make sure your nails are cleaned and trimmed before touching another man's penis or scrotum (unless you are a woman who has her nails done, then just be careful what you do with them during said deed).
I am sure the above two tips may seem silly and rather obvious, but I have found both of them are little things that can go a long way when trying to give a great blow job.