For receiving cunnilingus, I enjoy all positions (especially if I am sitting on his face or on my knees with him licking beneath me), but when we get serious (e.g. it's time to have an orgasm), I have to be flat on my back, legs straight/slightly spread open and held down by his arms, and I HAVE to be holding on to something behind me (e.g. bed frame is best). It's very hard for me to come unless I am actively pulling "up" and tensing my lower abs... hence, climbing the rope in jr. high really got me going.
I also buck like a wild bronco when I come, so he's gotta restrain me a bit in order to maintain contact.
As for fellating... I love surprising him beneath the sheets, when he's on his back, and spend a lot of time on his balls before moving up to the cock (and continuing contact with the balls). I also enjoy kneeling with him standing over me, his hands in my hair. Doing it in the shower (standing) is a lot of fun too, especially if he comes on my face.
Also if he's kneeling over me, and I'm beneath him (sort of fucking my mouth, but with my head propped up by pillows). That way, I have full access to the balls and he can come wherever he wants, though I find it more difficult to swallow in that position (that's when my gag reflex is strongest, since the semen sometimes shoots right down my throat). But I love watching his face in all circumstances.
Pretty much everything, I guess.
