Make sure you refer, in the main message, to any attachments you are adding and of course make extra sure that you remember to include the attachment(s). As attachments can transmit viruses, try not to use them, unless you are sending complicated documents. Copy-and-paste text-only contents into the body of the e-mail. If you use an attachment, make sure the file name describes the content, and is not too general; e.g. 'message.doc' is bad, but 'QA Report 2014.doc' is good.