my wife is difficult to get to orgasm too. i make sure she gets her's before i get mine, she takes a lot of stimulation--sometimes she can go just by having sex, but she has to be really excited before to do that. when we were married, she had never orgasmed. she was kind of a slut before we were married, she always wanted to fuck and did many guys before and during the time we were dating--but she never orgasmed.
now i make sure she does: a lot of breast and nipple play, caressing and fondling her pussy and clit, rubbing, and vibes can get her close then a good fuck will put her over or vibe her while she screams into a pillow and keep it going on her until she begs to stop. she didn't orgasm until she was like 25 or so, but once i got her to go, i made it my mission in the bedroom to get her screaming orgasms every time.
when you do finally orgasm, don't be afraid to scream, thrash around, and just go crazy--enjoy it... it makes it a lot more enjoyable for you and your partner.