1. If you do nothing else, make sure the return address is legible, and in a clear place. Then the item of mail will come back to you if there has been a problem.
2. Pay the correct postage (see http://www.royalmail.com/portal/rm/jump2?catId=400023&mediaId=11200122). A large letter with a 27 pence second class stamp on it will be given to revenue protection, thus delaying your mail and incurring a £1 handling charge (plus the postage deficit) for the recipient.
3. Sellotape! Not Scotch Tape: it peels off too easily. Sellotape. Or parcel tape. But not masking tape. Tape is used to stop the contents falling out of the envelope, so you should make sure that you use enough of it to seal all the edges of your envelope.
4. If you put too much into an envelope meant for a few sheets of paper—especially if the item of mail then qualifies as a packet—it will probably rip. If it rips, the contents will fall out. You should use an envelope that is more suited, perhaps a padded envelope.
5. If your ripped or open envelope is handled by a member of staff at the sorting office, and is seen to be ripped or open by that member of staff, he or she should use Royal Mail tape to seal it. Or it should be put into a "sorry we damaged your mail" bag. This seems to be done by a designated person; therefore your damaged item will sit on said person's table until he or she is at work again. Or, if there are no more bags left, it will sit on that table for days, ensuring your mail arrives both damaged and late. Only the conscientious among us will set aside open and damaged items; if you are unlucky, your envelope will arrive at its destination without its contents.
6. Writing "Please do not bend", "Please do not bend!", or even "PLEASE DO NOT BEND!" on an envelope that apparently contains a certificate is no guarantee that it will not get bent. Whilst it might prevent the post person folding it up to fit it into your letter box, it doesn't protect against it being bent by other items in the mailbag. An envelope with a cardboard back is a better option, though they are not completely bend resistant.
7. Don't use second class post. An occasional item of first class mail will get missed, and sent the next day, but second class mail can hang around for a long time, increasing the chances of things getting lost or forgotten about; best just to use first class post and get it on its way.
8. Packets should be well-protected on the inside, packaged in a sturdy envelope or box, and sealed very well. Wrapping paper probably isn't going to last long, so don't bother with that on the outside. Your return address is important.