Some tips for artists dealing with digital files

It seems like I see this a lot, even with very skilled and experienced artists.
Most art schools have digital production programs nowadays, but these might not necessarily be very comprehensive. Nor do all artists attend art school, of course.
So here are a few tips for artists scanning their work to manipulate digitally.

-Standard print resolution is either 150 or 300dpi. The higher the dpi, the better the quality of the print but also the larger the file. In Adobe Photoshop, resolution can be adjusted under image>size. Always start your images at 150-300 dpi, and work on them at that size.

-I'd recommend scanning everything at a minimum of 300dpi. You can always reduce the size later.
The basic formula is: something at a high resolution gets reduced, it looks even better. Vice versa looks terrible.

-Standard web resolution is 72dpi. That's just the optimum number for display. Posting something at a higher resolution won't make it look crisper, it will only make your image really large and make it easier for people to steal your work (72dpi is, obviously, far too small to achieve a decent print.)

-Keep a backup of your file, with all of the layers, saved at full resolution. Even if you never plan on printing your image.

-Save everything as a tiff or psd while working on it. Tiffs and psds are loss-less files. This means, they don't discard any information when they're saved in order to compress themselves. They're the most flexible of the loss-less files. Jpegs and gifs are not loss-less, which means that every time you open one to edit and then save it again, information is discarded again. Your image will get pixel-y and crappy looking every time, the colours will be off. They're the perfect format if you're just saving something for the web, because at that point you never plan on opening and editing it again, and if someone steals it they'll run into the same problems you would.

-If you're working on an image to submit to a client or for a competition, always send in a 72dpi image without layers for the previews. If you win/they pay you, then send the full resolution file (though again, unless it's in the contract they don't need the layers, just a flattened tiff, pdf or jpeg.)

-If you're trying to save an image with a transparent background, save for web>save as a png is the way to go.

-Images don't generally like being enlarged. If you want to enlarge something, scan it at 1200 dpi, run a threshold on it in photoshop (under filters) then go into image size and bump the resolution down to 300 or 150, while increasing the size (preferably by 4x or less, the less you enlarge it the crisper it will look.)
Another way to do this is by opening it up in Illustrator and vectorizing it, by going to object>live trace>options. I'd preview everything before finalizing it. Vectors can be enlarged endlessly, but the very process of vectorizing will screw with your image a bit, and totally mess up gradients. Be sure to play with it and see how close you can get to the original. Especially good for line drawings, though.

-Scanning in some line art, and want to clean it up really well? Not have to deal with that haze of white around your lines? An easy way to do this (that I discovered by accident).
1.) Bump up the black levels, to get rid of any grey that might have shown up in your lines during scanning. (another thing: brightness/contrast also discards information. It's better to adjust those settings by going into levels.)
2.) Make your background layer into layer 0. Make two new layers underneath it. Fill one with white, and the other with black (or whatever other colour you'd prefer your linework to be.)
3.) Go to Select>select colour range. Click on the white in your picture, and bump the fuzziness all the way up.
4.) Click on your black layer. Hit delete.

You now have your linework. It's a surprisingly clean looking method, far far better than the magic wand tool. Now you can clean your lines up further by vectorizing them in Illustrator, or just use them as is.

That's all. If you already knew this, yay! If not, I hope it helps.
I only learned the danger of jpegs just this past September. Boy am I kicking myself about that!