Step One: Buy a good printer. You'll need one that uses archival inks. Do your research on this. Here are a few printers that I found. I personally use this one. You'll want to find a printer that both uses archival inks as well as one that will continue to sell these inks. While most of these printers are expensive, there are inexpensive ones out there. You won't find one for $200, but you also don't need to shell out $2000 either.
Step Two: Scan your art before varnishing. That second part is just so you can avoid glare issues, but the first part is essential. This is hard if you make larger pieces than the scanner itself. For larger pieces you'll need to either fork over cash for the expensive and large scanners or go to a professional who has one. For smaller stuff, many good printers have good scanners. You'll need one that scans at 600dpi, which is double the standard of printing. For example, you can take an 8" x 10" at 600dpi and scale it out to 16" x 20" at 300dpi and you're good to go.
Step Three: Edit the photo. There are two purposes here. One is that this will allow you to fix any blemishes or levels that may have come out wrong in the scan. The other is setting the image colors for printing as opposed for on the Internet. Yes, this does make a difference (research this to be sure. I know it needs to be done with a camera, but what about a scanner?). Here is a great article on how to update your image for printing vs. viewing online.
Step Four: Crop your image. Pretty self-explanatory here. If you want to sell an 8 x 10 image you'll need to crop it to 8 x 10. I'll be honest. I do slightly larger (8.25 x 10.25) just to have a little piece of extra space for the matting.
Step Five: Mat and Frame your print. I think this is one of the most important steps. A lot of people don't bother, and I can see the value if you're selling cheap, but I find that it does make a difference to mat your print at the very least. Putting a white mat around it and into a plastic sleeve makes it look more professional. However, framing your print will really add value. In fact, I think the right frame could even double the cost of the frame and mat themselves.