Czech long vowels

Slavicists,
Are there any rules for predicting (from polish, russian, proto-slavic or
so) where there is to be a long vowel in the corresponding Czech word. I
can spot some straght off, like the -ie neuter words and -ej fem. decl.
ending are obviously contracted and thus motivating the long i and e
respectively. But to be sure there are more rules ...

Harald