I personally use the windowbg colours as a gradient for the head/base images.

You can do a lot of different styles with filters (assuming you're in an advanced imaging program [photoshop, paintshop pro, etc]). Some people say that you should always do rounded head/base images, but truthfully, do what you think flows.

If you're going for random, do something like this:
head image - left corner rounded, right corner diagonal
base image - left corner straight, right corner rounded

In the head image, a lot of people like to put a little spot where the category image goes, this is perfectly fine, it's completely up to you whether you include this.

Another thing you can do regarding colours is to take some colours used in the skin and lighten/darken them a bit to add to the contrast a bit. If you've got a dark blue/black template, you would most likely want a gradient going lighter colour to darker colour.

One thing you can do to make head/base images cool is to add little techy lines etc etc to them, assuming the skin is techy. Use layers. Obviously try to make the skin match. Just have fun with it; go wild.

Sorry for the long post