For my Tsunade I hid a small hook at the lowest point I wanted the top to open to, this way it'll stop no matter what I do. If you are very careful where you attach it a very simple hook and eye will prevent costume malfunctions very nicely.
~I've gone to look for myself, if I should return before I get back keep me here!~
Costume list 2009: Finished
17 initially planned :: 15 completed :: 6 moved to next year
Costume list 2010: Started
6 from '09 :: 9 initially planned, now up to 22 total