Firm and fast sucking and stroking are very pleasurable. Your partner will let you know if you are too rough, but many guys tell me that women tend to be too gentle. Tease and talk. You will discover the technique that works best for you and your partner. Some people start off slow or soft and get more aggressive.
Some people start and then stop and then start again.
The purpose here being to build up the excitement. You don't have to perform oral sex to orgasm. Especially since some men do not reach orgasm from oral sex alone. It can be one part of sexual play. Do it as long as it feels good, then move on to something else.