Well, since I use photoshop, I don't know what the difference in controls are between CS4 and GIMP, but here's what I do:
[spoiler]
1. Create an all black bg of desired size.
2. Using size 5 and 9px brushes make several white dots on the page to serve as stars. Try to vary brush size so that it'll look as though the stars are from different distances.
3. From here you have two options. First, you can continue with step two until you have a star field you like. Second, you can find a good stock photo of a star field and blend it with the stars that you created. I like to do the second when I'm creating a nebula, just because it's less tedious and I can get right to the lighting and coloring effects. The key with stars is that there is no uniformity. It isn't like static on the television, some stars are super close, some are far away, some are clustered, some aren't. If you find your sky looking like it has too much of a pattern you need to go back and reconsider.
Nebula Cloud Effects:
1. Find a stock photo of a cirrus cloud. In a separate document desaturate it and adjust the levels as needed.
2. Copy and paste this stock photo onto a new layer of your original background. Set layer to 'screen' and opacity to about 15-20%.
3. Directly above this create another layer and, using a large soft brush, brush in a pastel color. Set this layer to 'soft light'. The effect should illuminate the clouds.
If I can find it, I'll post a link of the original tutorial that I used for making the planets. I've modified some of what he said to suit my needs, but it's an awesome tutorial overall.[/spoiler]
Hope that helped!
