I was looking around the turning board this morning, and happened upon Vaughn's post on his first hollow form. He has turned a rather nice piece for a first form. I could suggest that the collar could use something, but just cant put my finger on just what it would be.
I have learned by doing tons of reading and playing a bit on the lathe that when turning a hollow form you never want the major diameter of the piece to fall at the equator of the piece. It is most desireable to have it in either the upper third, or lower third of the piece. The foot of the piece is normally about 1/3rd of the diameter. (This is very subjective, there are many trains for thought on this.)
A good tool to have at the lathe is something that probably no one has even thought about having. Nothing more than a simple fine chain like you would wear around your neck. If you grab each end of the chain and play with raising or lowering one end or the other, these are the shapes that we should strive for in our turnings. A chain simply cannot make a sharp point, or a bad transition from one element to the other.
Y'all must understand, I am not trying to preach gospel, or say that what I do is the way you "HAVE" to do it. Its what works for me.
A good exercise is to get some branch wood in the 3"-4" diameter. Practice making your forms with this wood. You dont even need to hollow it, just make forms. If you can nail the forms on small pieces, making larger pieces will come much easier for you.
One of the better small form turners that I truly enjoy watching is Carol Valentine on the Wood Central site. Look over her work, and this can give you a good idea of what to strive for.
Sorry to be so long winded. Lets try and kick this topic around some and see where it goes.