A lot of thought - maybe too much thought - goes into trying to make magic every time you have sex. We'd all like to think we're sexy and good at it and one way to measure that is whether the partner "seems to" enjoy himself and whether he gets off.
I've tried a couple of different things over the years.
1) told the guy I didn't feel like getting off but that what he was doing felt awesome and I just wanted to enjoy it for a while.
2) helped the guy find a position that works for you. go about it playfully, tell him you'd like to try some new ones out to see how they feel. I don't know all the funny names for the positions, but I was with one guy who tried several before we found one that pushed my buttons (in a good way).
3) if this is a general issue for you and you want to cum, then ride the wave the moment it starts. What I mean is, that some guys are into "edging" - bringing you close to cumming and then backing off over and over. That's probably not going to work with you. Better would be to find what's working and take it all the way to orgasm immediately.
4) don't masturbate for a few days beforehand. The chances of getting aroused to the point of orgasm are better for me after a few days of abstinence. After a week I'm practically a bitch in heat! (but please forget I said that)