I usually just send an email file. If they want a print from me, I'll do it, but it's more expensive since I have to pay for that and shipping. If they want a print, they can go down to a print shop and pay for it there.
Talk to the customer beforehand to figure out how they want to get the art. Some people just want it posted here, others will want the file emailed to them in a quality suit to print.
Generally, I send a high quality .jpg, a flattened photoshop file, and, if requested, lineart on a transparent layer they can have colored.
Basically, if it's done over the internet, if the client doesn't demand a signed contract or anything of sort, then you give him your price and Paypal e-mail, then you discuss the image concept, style, deadline, etc. Always ask for half off the payment in advance in order to get started. Once you receive that, you work on the image and send progress shots to make sure the image is heading in the right direction. If it's pretty much up to your creativity, then progress shots aren't always necessary. When you feel the image is complete, show the client a low res version (below 1000px at 72 res). If he's satisfied, then he/ she pays the second (final) half of the payment and you send him the the completed full resolution image. Ask for the client's permission to upload your work online, if you're not sure. Always remember to get payed in advance before starting otherwise you're risking wasting your time if the client doesn't pay. Hope that helps. Let me know if you have any more questions.
When I am looking for someone to commission, I will look in the Forums, or do a search on the main page for "Commission girl" or Commission whatever so I can see who is taking commissions in the last week or so. if they have uploaded in the last week, then I figure they are still probably doing them.
I sometimes look in the Commission groups, but they don't seem to be updated very regularly.
If you are going to advertise in the forums, then I suggest you learn how to add a few pictures to the message, as if they are hosted off site, then I have to make do 4 clicks to look at it and then go back to where I was before, and if there are about 20 new messages from people wanting to do commission it takes time to look at them all.
In some cases the artist has sent me two copies, one with a watermark, and has asked if I would use that one on my blog, so people can't easily take it and reuse it for themselves, and I am quite happy to do this.