How to determine the orbital hybridization?

Are there any _formal_ rules which can determine the hybridization of an atom?
For example, for PtH2 I can write different versions of platinum hybridization: sd1, sd5.
How to determine the right one?

I have the impression that for d and p elements the rules are different. But if p described in many textbooks, for d usually only examples are given, but not rules. Are there any?