In her piece “How to Graciously Say No to Anyone,” Alexandra Franzen suggests the following: Thank the sender for thinking of you, decline, and, if you can, offer another form of support.
We agree to many requests not because we want to do them, but because we don't want to be seen as rude, arrogant, or unhelpful. Often, you have to consider saying no to someone you will interact with again in the future—your co-worker, your spouse, your family and friends.