I don't know of any research to back this up, but in my experience, there are three things which help:
1. Don't fap or have sex for a while. If it's been a long time since you've had a sexual release, it will feel better and it'll be longer.
2. "Edging". This can be done both when masturbating and during sex. Basically, you get to the point where you're about to cum, then stop for awhile until you cool back down. Keep doing this over and over. The more times you do it before actually orgasming, the longer and stronger the orgasm itself.
3. Sexual Stimuli. It seems to matter how you get off. For example, fapping vs sex is a big difference in strength of orgasm. But even within those categories, there are differences. For example, if you have sex with a girl who you're particularly excited about, the orgasm will often be much more enjoyable.