Breathing deeply and staying present rather than inside your head makes a massive difference. Anxiety kills orgasms — learning to quiet the mental chatter is the most underrated advice.
Edging — getting close then backing off several times before finishing. The intensity when you finally let go is on a completely different level. Requires patience but totally worth it.
For men: learning to separate orgasm from ejaculation through tantric breathing sounds very new-agey but actually works. Multiple orgasms without refractory period are genuinely achievable.
For women: consistent pelvic floor exercises. Kegels done properly and regularly make a real measurable difference in intensity. Takes about three weeks to notice but keep at it.
Slowing down in general. Most people rush and it's the biggest mistake. The build-up determines the peak — spending twice as long getting there is almost always worth it.