One thing I've heard mentioned a few times recently by other artists is how working digitally has affected their ability to make artistic decisions. The common complaint is that, while digital art programs like Photoshop and Painter are great for working fast and freely, the ability to endlessly revise a piece with the help of the "Undo" command and layers makes it that much more difficult to accept the artistic choices you've made and move forward with a piece. If this sounds like you, then I've got a few tips to hopefully get you moving forward more confidently with your digital art!
Do separate thumbnail sketches.
Sometimes it's more beneficial to have a lot of small thumbnail sketches to look at and compare side by side. You can quickly pick out things you like about different sketches and move forward from there instead of endlessly revising one sketch and hoping that what you end up with is the best version of your vision.
Lower your number of History States.
In Photoshop, this can be done via the Preferences menu (in the "Performance" tab as of CS3). By lowering your History States count from 1,000,000,000 to something like 20, you'll force yourself to think a little more about what you're doing as you paint because your "safety net" of undoes will have drastically shrunk. You'll learn more by painting over your mistakes rather than erasing them from time. A lower number of History States will also improve the overall performance and speed of Photoshop.
Merge down your layers
Typically once I've painted a section that I like and feel good about, I will merge it down to the canvas (the bottom layer). Psychologically, it's a way for me to acknowledge, "Okay, this is the best I can do for now. It's time to move on." I call this "Marrying the Canvas," because sometimes you just have to commit!
Make use of good reference!
This is just a good idea in general, but especially for digital art. I can't tell you how many times I've painted and re-painted and re-painted and re-painted something without getting it right because I allowed my digital tools to become a substitute for getting good reference images. Not only will this not help you learn, but it can really leave you feeling much more frustrated than you need to be. Don't fall into this trap! Know what it is you need and go get it!
These tips may not be entirely suitable for certain projects where revisions and changes to final art will be required very quickly (like in concept art), but for illustration and general painting, they can help you take more confident strides towards calling a piece finished and accepting it as it is.
Happy painting everyone!