I am not an expert, but since i am in the industry and all i would say is planning is a key thats most missed. Many start to model and do so much without planning..

If you want to work with Mental Ray then either polys or nurbs will be the way.

You can convert your sub-d to nurbs and then just adjust your way out,, which i think is a pain..

Or just live the object and draw the curves and patch it.

Ok as far as the scene goes.. scale should be set. I work real scale.. or some weird scale that i see fit.. But real scale should be good as it works better with effects and so on.

Lastly the scene. Actutally the background is rendered on a seperate file.. and the ground near to the character and some items that the character is interacting is put together with the character on another file..

If you are not animating the background.. then think.. do 1 render or 10000.. Hehe ok the problem comes with you are compositing.. or puting everthing together.. Thats always a struggle that you have to deal with. Taking notes on perspective.. camera settings. and other things will help.. take not help and not clear your problem.

Plan ahead and enjoy the ride through.

Kumar.
VFX - Trinity Television