Having a quick look at the link TvF posted, another good reference came to my mind.
On belleslettres.eu, there's a good article (in German, including video) on where to place a reflexive pronoun.
The most essential information are already given in the other answers. The reflexive pronoun is usually placed behind the verb. This is valid for a simple sentence like:
Er hat sich (nicht) verändert.
This, of course, can't be true for questions and subordinate clauses in which - as you know - the verb is placed at the end of the sentence. In these cases the reflexive pronoun follows the subject.
Ob er sich geändert hat?
Es hätte mich auch gewundert, wenn er sich verändert hätte.
Having said that, it is much more natural to place the reflexive pronoun before the subject in case of the subject being a noun:
Ob sich Peter geändert hat?
Es hätte mich auch gewundert, wenn sich Peter verändert hätte.
The rule is given in the link I provided. Subject before object; but, in case of having a mixture of reflexive pronoun and noun, place the shortest word before the other. Since the reflexive pronoun is shorter than the noun it precedes the noun.
It doesn't sound odd, however, to place the reflexive pronoun behind the noun. It even can be happen deliberately for emphasis reasons.
And last but not least, but this is also given in the link above, in case of ambiguity you need to put the reflexive pronoun behind the subject. Compare:
Ich habe gesehen, wie sie Peter geschlagen hat. (missverständlich)
Ich habe gesehen, wie Peter sie geschlagen hat.