I have gathered the following information from the University of Michigan's German course:
Normally the reflexive pronoun will be accusative. If the verb already has an accusative object, however, then the reflexive pronoun will be dative. Note that you will only notice this difference in the ich- and du-forms, since for all the other persons, the accusative and dative reflexive pronouns are identical.
This seems pretty useful guide for me. Can I take this as a rule when determining whether 'sich' should be accusative or dative? Is this rule really valid?
For example, sich überlegen. This has to be dative because you need to consider something and it already has an accusative object here.