How to add a soft fantasy glow to your art
Using a soft brush with a low flow works nicely to apply glow around highlights, and I recommend doing this at the final stages of a painting. If you really want to achieve a global glow effect there are simple shortcuts, but know that this will change the colour values in your picture quite a bit. Unless, of course, this is the effect you are aiming for.
Add a flattened copy of your picture into a new layer at the top layer. Now open the Gaussian Blur dialogue (filter/blur/Gaussian blur…) and drag the radius slider until all details gets blurry. Now, go to edit/fade Gaussian Blur (Shift+Ctrl+F), leave the Opacity at 100 percent and set Mode to Screen, and click ok. Do the same routine one more time with Gaussian Blur, fade Gaussian Blur, but set the Mode to Soft Light this time. I am guessing the picture has got too much soft glow right now, so now you can blend the amount of glow you want with the original picture by sliding the Opacity slider on the layers menu.
An even easier method is simply setting the blurred layer to “Linear dodge (Add)”, and then adjusting the opacity until you’re satisfied. For an even better result, up the saturation on the colours of the blurred layer.
Create a simple soft glow with a Gaussian Blur and Fade Gaussian Blur routine, but be careful to not overdo it.