Cases and Prepositions (amongst others)

G'day y'all, I just have a few questions for you.

In my conlang, tentatively called Ajuk, I've got seven cases:
Nominative, Accusative, Dative, Genitive, Vocative, Ablative, and
Instrumental. I've gotten to the point where I have to divvy up the
preposition structure, and I realize that each in a language with case
the nouns in a prepositional phrase have to go into a certain case,
depending on the preposition and the meaning intended to be assigned to
it.

At first I was intending to make a system like in German, but that only
covers four of my seven cases. The Vocative doesn't neccessarily need
any, because it is fairly limited in it's use.

So what I want to know is, is there any particular system by which the
prepositions are divided up in the case structure, or is it different
from language to language, even in languages containing the same cases?

Another thing, I've got pronouns that mean things like "for that
reason", "at some time", "in this manner", and such, and I need to
decide what case they would go in. This kind of ties into the
preposition structure, for example a word meaning "at some time" would
go in the same case as nouns in a prepositional phrase with a
preposition meaning "at". Again, is there any system to this throughout
several languages or do I just have to make my own?

Anyways, thanks for your time, and I look forward to an answer.

--
Robert