There are mistakes you will regret forever. Among them are a lot of brainless actions that pertain to your artwork. I will sum up a few for you to avoid. Fortunately, I haven't made all of them myself (yet) but also collected a few from other artists.
Save a lot of files.
Don Seegmiller says he saves up to one hundred or more per painting. If your disk space is limited, you can later burn them to DVD, but you'll be thankful to have a file to go back to if all goes wrong. Save it externally, too, if you can.
Keep those saved files around.
There's no characteristic pigment or stroke to a piece of digital art. Anyone could have produced what you have. The only safe way to prove you are the original creator (in court, if necessary) is to keep files in working resolution, with any layers and whatnot. What the negative is to photographers, that original file is to you.
Always save the copy first.
Before making destructive changes like cropping, always save a new file first, or you will inevitably run into the "save changes?" dialogue, and out of sheer use, you will hit "yes", and hours of work go down the drain.
Use non-destructive methods.
Using layers is called non-destructive because you can remove them whenever you please without wrecking the whole. If you don't have the option for whatever reason (slow computer won't take many layers, program doesn't offer any), save as a new version. First.
Note: many artists will tell you to use only one layer. This will allow fewer changes but is a good artistic exercise.
Bookkeeping is boring, but keeping your license agreements in order will save you pain. I have a lot of "usable for personal use", "usable for non-commercial purpose", "usable if you credit me" things and others, and I need to keep track of whom to credit with what. It's useful to rename downloaded, say, brush sets by adding the creator's name to find them later. Or only use resources that are completely free to begin with.
Save to other format.
Earlier versions of Painter used to eat my paintings, so I saved them in bmp format as well, and this saved me a lot of tears. Should your format seem unstable for whatever reason, save it as a different format - bmp is lossless, so it should be your first choice.
A word about size.
Business pros like Feng Zhu create artworks ten thousand pixels wide. Ten thousand. Use the largest size your computer can still handle. It's a tried and proven method to sketch on a small format, then upsize it when you don't need giant, fast brushstrokes anymore. When working for print, make the file a bit larger than the final printing size, because upsized prints look terrible, but downsized ones very nice.
Learn about printing.
There's no room here to discuss the technicalities of bringing stuff from a backlit screen to a printed paper that uses a different colour space. But you should find some guide to printing; print your artworks yourself or have them printed and compare until you find out how to set up your files best. Also calibrate your monitor. There is a calibration help on ballisticpublishing.com, for instance.