How to make a soprano out of a tenor?

After the shark joke, how can I raise the pitch of my tenor voice to sound like a soprano?  I want my back up chorus to sound female not male.  I belive that shifting the pitch up an octave will do the trick.  I tried the octave harmony in Izoptope's Nectar, but found a lot of artifact.  I am going to try using melodyne next.  I think I have heard of otherwise to change pitch and/or timbre to accomplish what I am after.  What ideas might you have?

Trax by Flux. Pricey but it's pretty darn good. No guarantee on the artifacts but you might at least demo it first.

I've tried both Nectar and Melodyne approach. If you have a good clean take, the Nectar should work...make sure the setting for the voice is medium or high. I open up the Mel standalone and just copy/past the specific blobs and shift to whatever harmony I'm chasing. I think it's better that Nectar with less chance of artifacts. However, taking it up and octave might introduce the latter. 
 

Pitch shift and formant shift are not suppose to be the same. If you raise your voice by octave, the formant shift should be much less. In V-vocal it's like this: transpose pitch one octave up, then set Formant Control: Pitch follow=0, Shift=30~40.
 
Try to keep recording level as high as possible to avoid artefacts because V-vocal does not recognize quiet audio.
 
One more thought. To avoid extreme transposition try to record your back vocals in higher key. Let's say your tune is in A minor. Transpose the instrumental tracks to Cm or Dm and sing backing parts. That way you need to transpose them only by 6th or 5th, not the whole octave.

dilletant
Pitch shift and formant shift are not suppose to be the same.

That's actually the point when creating backing vocals. With the formant setting you can change the
voice so, that it's like another person singing, even when you don't change the pitch.

I agree about singing in a different key so you don't have to shift a full octave.  You can copy one track (like piano) transpose it up a few steps, mute everything else, and sing your part with the piano (or guitar or whatever) as your guide.  Then you can shift your voice up less than an octave and tweak the formant.
 
I doubt you will be 100% happy with this, but it all depends on the individual's voice.  It may work great for you.
 
It is amazing how many female singers will be happy to help with some studio work for little or no money, just because they enjoy it or want the experience (or because you're friends).  I'm sure you've thought of this, but keep in mind that the female singer does not have to be perfect because you can fix her part with Melodyne.  Better to tweak her pitch a bit than shift yours a whole octave.  And you can always lower her formant just barely to make her voice blend better with yours.  This isn't what you're asking: it's just an idea.