photo shop cs

if you show a picture it would be better ... There are a bunch of PS tutorials on the net... usually heres a basic concept.

- Use the freehand outline tool to trace the part you want to enlarge.
- once selected select copy
- now paste as seperate image to have a backup just incase, paste another one if required.
- any of those image you can use to enlarge to a certain percentage
- paste the large cut-out back to the original image as a layer so you can adjust it.
- lastly use your eye to align properly, use smooth, smudge or push in any order to achieve a realistic look

this is just a basic, read more about it in PS tutorials site... although its not specifically to enlarge a botty, you can get an idea..

You could also use the liquify feature to drag it out manually but this doesn't always give the best results.

I agree with demonhale for the most part; don't necessarilly try to go all the way in one shot, if you want the most realistic look.
bring it up in 3-4 steps, there's less to do with less loss.
I use both PSP7 & PScs
PSP7 = speed & user freindly, but distorts with each resize, rotate, stretch ....
PScs = QUALITY but less user freindly