Here are some "guidelines" that I believe will assist in reviewing.

1) Go through the whole thing at least two times before reviewing. First time to check it overall, second time to check specifics.
2) Make sure to say what you like and what you don't like.
3) Tell people what's good, compliment them, but make sure to focus on what can be improved. Everybody can use constructive criticism, as we can all improve.
4) Be kind. Don't insult them for making a mistake.
5) Make sure that it is a reasonable length for the song. If the song is 13 seconds, obviously you can't make a large review. If it's over 5 minutes, you can do a long, in-depth review.
6) Only say what can be said about it. If there's something without many compositional elements, or mixed badly, you can comment on it and say what might help, but you can't really give much fine-tuning advice unless it's fixed. Ever tried to juggle four eggs when you can't juggle two?

These are my guidelines for reviewing that I attempt to follow. Take them or leave them. ^_^