Refusing a review request - politely
Today I received a review request that was peripheral to my own research area, and which comes at a time when I am too busy. Nevertheless, I did not want to discourage the journal editors, and sent the following reply:
Thank you for the review invitation.
Your journal is interesting for me. Unfortunately, I am not very familiar the subject area of the paper, and am currently too busy with other writing and editing obligations.
I am happy to consider other papers for review in the future.
Yours sincerely, [etc.]