The short list theres a huge one in the archives somewhere, search deep 1 year old id say...
Check norrmals (they must be consistent, use conform, and their direction)
Check for nonmanifold surfaces.
Check for holes (confuses the algorithm)
[li]Check history (might confuse never noticed tough)
There was some other things too cant remember tough! Should fix 99% of cases.
forgot:
[/li][li]Use closed surfaces when apropriate, works better.
[/li][li]Make sure there isnt invisible, polygon gaps (merge vertices and edges first).[/li]