Ah yes I was stuck on advanced level 15 for a while, a toughie! Can do it almost everytime now, the key being to keep up some momentum and make smooth turns. Got up to level 28 last night as I seem to have shot up a level in my ability. Expert is a proving to be a different kettle of fish though, might get to level 4 tonight
For truly amazing SMB skill though, check some of these videos out, they blew me away and inspired me to keep practicing!
http://www.dragoneye.ca/gamerfeed/monkeyball/