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.