Variation is of the essence: sucking and licking can be done in many ways and on many places; the penis can be held in different ways; your head twisted in different directions.
Ask your lover what he likes, don’t be shy. Guys have different preferences. And remember, good sex and good communication come together. Also, be attentive to his body language.
Use your hands to increase his sensations. You can massage his testicles, pinch his nipples or stroke his anus, while giving him a blow job.
 If your mouth gets tired, use your hands. If you also use your hands while giving him a blow job, the transition may not even be noticeable.
Be aware of your own sensations. Use the head of his penis as lipstick and feel how soft its skin is.