There are two recurring problems I've always found myself facing when dealing with digital art. I'm sure there are a ton of ways to deal with them, but here are a couple of ways I discovered.
The first problem is lightening up dark tones. A lot of cool backgrounds and textures are also, unfortunately, on the dark side. Obviously this has something to do with how you set your computer screen, but I like light in my pics (unless the pic has a dark theme, which is a different story).
I discovered a way to deal with this totally by accident. I was preparing a pic as a gift for an online friend for her birthday. It looked dark to me, so I made the bottom layer a little transparent and put a white layer beneath it. The problem with that was that other layers on top of the bottom layer looked darker than the bottom layer. I didn't want to make those transparent (can't recall why but I think there was some issue).
I removed the transparency on the bottom layer and saved the pic from GIMP's default .xcf to a .jpg. That file had two layers: the combined layers of the .xcf file and a white background layer that GIMP always inserts into a new file by default. I still thought it was dark, so I started playing around with the pic's transparency. Because there was a white background underneath and no other layers in between, this white background added light to the entire pic.
I also discovered with my latest pic that you can use a white or textured white layer over the pic and play around with the transparency to lighten it up a bit. It depends on whether you want a little texture there or just want some brightness to show through from underneath.
The second problem I constantly encounter is making pasted layers, particularly of people, blend into the rest of the pic. I hate the glued-on look. I stumbled upon the smudge tool. It basically does what it says, smudges the area you brush over with your mouse. This, of course, distorts the colors there, but it works nicely for images in underlayers.
I wrote a post on my blog, for instance, about Joseph Campbell's Hero's Journey on the meeting with the goddess
and added a pic. In order to get the head of the menacing woman to fit nicely into the background, I had to do a lot of erasing. I chopped off her neck, a lot of hair, and even an ear.
The smudge tool really helped remove the sharp outlines from those changes and blend into the background. You can, of course, notice distortion if you view a big version of the pic, but it's invisible in the post. Shamanic Transformation into Nightbird