I start with a grocery bag cut to size and stitch it together. Sometimes I put a little tape on the corners to reinforce them but you really don't need to. For the labels I use some Lotta Jansdotter post-its I bought a few years ago and cover them with clear tape (after the address goes on).
This is what I sent out to my partner Tacha for the Secret Santa Swap last Christmas.
If I don't need to make an envelope I have this cute tape by petitcollage I get to play with. They sell the tape in Portland at Presents of Mind.
The flower is from my new Lotta stickers.
I have a pouch swap I'm finishing up and will get to wrap soon. Wouldn't you know it there are lots of fun groups on Flickr all about mail art if you want more inspiration.