It is a great honor to be asked to be Maid of Honor so the decline should be a good one such as: lack of funds if you are expected to pay for your dress and shoes and extras; illness in the family such as a parent; traveling too far; no time off work for the event; etc. You should phone the Bride and not email them or send them a note as that is too impersonal. Phone the bride with your excuse as to why you have to decline, but be sure and tell her you were honored she chose you. If there is a good excuse such as looking after a sick parent, etc., then tell the bride you will make every effort to at least be at the church or miss the church and be at the wedding reception. If this is not possible then hopefully the bride will understand.