"No thanks."

"Im not in the mood,maybe tomorrow."

I have a headache.....she'll identify with that one.

"You don't want to make the relationship weird by bringing sex into it"

I dont want to ruin our friendship by doing something we may both regret later! i hope you can understand this and still be friends with me!

Ah well you could maybe tell her you're not into that sort of thing due to (insert suitable offending reason)

thats a tough one...feelings may just end up getting a bit hurt...but you can lessen the blow by being completley honest as to why you dont want to. if anything she will see that you respect her enough to be honest and open.

i agree with ehedges say i dont want to ruin our friendship by doing something we may both regret later i hope you can understand this and still be friends with me

Well, you could lie and say you're dating someone or have feelings for someone else, or are gay. I normally don't say this, but this time, honesty might not be the worst move. I've read the above, so I know the situation better. I was in a similar situation myself - a female friend (happened to be an ex) that wanted to have sex. The thing is, I was and am attracted to her physically, and maybe if she had wanted to sleep together a few months earlier, I would've gone with it happily. But I had really finally come to see her as not an ex-girlfriend, but as a friend, more like a sister, albeit a sister I still thought of sexually, if not romantically. So I told her that it wasn't that I didn't still find her sexually attractive, and that I didn't want to sleep with her, but that it would risk the friendship. I told her I loved her and valued the friendship too much to risk it for anything, even the extremely incredible sex we'd certainly have if we did anything, and that that should mean anything, me being a borderline sexaholic. Again, I think honesty is often the very worst policy, but this time, your "truth" is actually not a bad truth at all. Go with it. And even if you don't actually find her attractive or want to sleep with her, maybe try to add that part anyway, as it will couch the blow and make it less about her looks in her mind.