Remember you have been asked because the editors trust your judgment.
Refereeing is a reciprocal activity. If you expect to have a paper published in a journal in the future it is a good idea to referee for it in the present.
We also expect people who have published with us to referee for us.
There is an etiquette to refereeing – if you write something sarcastic or nasty (always possible if you are frustrated/infuriated with a paper) do take time out to think how you would feel if you received such comments.