I agree with Perna with respect to the 'learned component'.

I guess there are a few things you could do... One of them (to work towards someone else being able to give you an orgasm) is to do everything the same as you do but guide his / her hand. I think that there is a bit of a learned compnent to positions and the like. My legs like to be rigid too :-)

Another thing you could do is some of the 'sensate focusing' stuff that I found on Wiki. Male sexual dysfunctions are often treated that way and female sexual dysfunctions are too (though it is of course controversial whether not having an orgasm constitutes a 'dysfunction'). The purpose of those exercises is to take the pressure off by having an aim of experiencing pleasure rather than experiencing orgasm. It is a way of learning (with your partner) what you like and what your partner likes too (with respect to both giving and receiving).

Sometimes orgasms can be a little like happiness... If you search for it directly then it always will elude you (too much pressure) but if you take the time to enjoy the moment (with the sensate focusing stuff) then it will just happen... Eventually.

I used to need to have my legs rigid. Then I learned (for myself and via a partner) to be able to vary the position slightly and clench my groin area. Turned out that clenching the groin (lower abs etc - I'm not so good on the precise details of the anatomy there) was what I really needed to do and that basically making my legs go rigid seemed to work because a by product of that was to clench those other areas...

Once you get the hang of clenching those internal and external midsection muscles (which I'll admit I haven't quite and I am SOOOO out of practice) then you can vary the position rather a lot... At least... This seems to be what started to happen with me...

Practice practice practice... good luck!