if you understand what russian word 'Куда' means, you should not have problems with Accusative. You use accusative every time, you are going to say Куда, and know, that 'на' preposition to be used.

Acc: Мы попали (куда?) на собрание, (на бал, на корабль, ...)
Prep: Мы оказались (где?) на собрании, (на балу, на корабле)

You may also notice that endings are different.