My technique that I have been using from before I can remember - as a toddler even I imagine, I can remember doing it as far back as age 4 - is I cross my legs (clothed is fine) and rub/grind them together. I get into a sort of rhythm (like two beats on the squeeze and then one on the release always seems to work) and I can come in about 5 minutes this way. It can be done in public - since I can do it easily fully clothed - but of course this is risky.
I can also come with the showerhead or a vibrator but this is my basic technique that I always come back to.
I have never been able to orgasm from manual clitoral stimulation, mine or a partner's hand.