I ship using USPS Priority shipping because they provide the boxes/packaging -- that in and of itself looks consistent and professional. I also take advantage of their online postage printing -- a printed label also looks more professional than a hand-written one. It can also help eliminate errors in addresses.
For the product itself, after some trial and error, I've settled on a system that works for me. Zip-top bags for each item (to prevent items from scratching each other), bubble wrap and colorful tissue with a branded sticker. It's simple and functional and doesn't cost a ton (you have to consider all of your packaging part of your expenses. After time, it definitely adds up!).
I've also started giving my jewelry a final polish before packaging -- seems like a no-brainer, but when you are tired at 1 am, it's something I usually forget to do! Smudges, fingerprints, I try and make sure those are cleaned up before the item gets bagged. I like using a Sunshine Cloth, but Pro-Polish pads are great, too (just remember that there is an abrasive on one side).
Got packaging tips and hints? Post a comment and please share!