I've got one.

























Just kidding.

Matching seams is just a matter of using a UV map for the body and some trial and error. You can get the maps at the first link on the wiki page here: http://wiki.secondlife.com/wiki/Clothing_Tutorials

My single best tip would be to use local textures, so that you don't have to keep uploading. Open the texture picker, choose "local", and use the Add button to pick the texture file you're working on. Now you can have a local texture in your skin/tattoo, and SL will update you anytime you save the file again. Saves steps and upload fees!

It's actually pretty simple if you're willing to put the time into it. I made a texture for the :ravenoid: - Nohemi's Big Chubby Body for their renamon av, albeit if you were to see it off the avatar, it'd look like a mess >.<

But biggest tips would be to use the temporary upload feature on some viewers constantly, that really helps on seeing how it'll look.
Just do a bunch of "for fun" side stuff, make something insane looking, overly concepual, and downright funny. You're going to go into your big projects in a more relaxed state.
Learn off of others work, if you can, download the texture and view it without it being on an avatar, you'll get an idea on how things should look.
And above all, don't be afraid to do something that isn't by the book. So what if you've got a bunch of splotches on the side or a series of weird shapes over the avatar. It's your work, you like it and thats all that matters.