Well, sometimes it depends on the cap I have as to what I do to it to get it that way. Typically, I crop the pic how I want it, then resize it to 100x100 in Paint Shop Pro using a bilinear resample. Then I sharpen it once. After that, I tweak the brightness and contrast until I like the look of it. I also find it helpful to add a light color filter set on soft light, especially when I want to bring out the background. I also tweak the saturation and color balance until it looks good.

Other than the initial resizing and sharpening steps, the rest is just trial and error until I like how it looks. ;D