Start with a base image. Mine is icon-sized but you can do this with any size graphic. Color it to your choosing. This will work best if it's not too light.
Now for the light/glow effect. Make a new layer. Leave it on normal 100% opacity for now. Choose white (#ffffff)as your foreground color. Choose a soft round brush, smaller is better when working on an icon-sized canvas (I chose 9 pixels). Go over the parts you'd like to have a little glow.
Yeah, that's not so pretty. We need to blur it for some subtlety. Go to filter > blur > box blur. The higher you adjust the number of the radius the softer the effect will be (I chose 6 pixels). If your graphic program doesn't have box blur you can use gaussian blur instead.
Now you can simply set it to screen and adjust the opacity (I set it at 44%) for a simple subtle effect.
You can go for a more dynamic effect and set it to softlight.
But that just punches it up a little bit. Duplicate that layer (with the white on it) and set it to screen. Then you want to adjust the opacity and/or fill (located just beneath the opacity slider) so that it's not that dynamic (I set the opacity at 50% and the fill at 45%).
You can achieve various degrees of glowiness simply by playing around with these settings. You can also use color instead of white. You can get some very interesting results by layering several different colored blur layers on different opacity and settings. And black in place of white creates a soft shadowed effect. Have fun and experiment.
soft color layers> bright color layers>