Overtone's Airbrushing in Photoshop
In this tutorial I am going to teach you how to create some of the airbrushing I do on my abstract pictures.

Step 1. Open up a render you would like to airbrush on. A render with spikes and crevices are usually better to brush on than round renders, but since I don't have a spikey render on hand at the moment, I'll just use this Bryce one.

Step 2. Press 'd' and 'x' to reset and switch your colors. Your foreground should now be white. Make a new layer and select your brush tool, and set it to around size 45 and a flow of 100%. Click and hold for a few seconds where you want your light to emit from.

Step 3. Filter > Blur > Gaussian Blur @ about 2.5.

Step 4. Repeat step 2 with a brush size of 60 and a flow of 20.

Step 5. Repeat step 3 but with a Gaussian Blur of 3.

Step 6. Change your brush to size 10, 100% flow. With your brush selected, press 'F5', and a window should pop up. Click on the text that says shape dynamics. Where it says control, change it to Fade, and then change the number box next to fade to 200. You may now click out of the window anywhere.

Step 7. Make a new layer, and select your pen tool. Make sure at the top that the icon with the pen (paths) is selected, and not fill pixels or shape layers. Then, click with your pen in the middle of your glowing light source you made in steps 2-5. Then click where you want your first curve to end, but do not let go but move your mouse up and down to change the curve of the line you made. Then click where you want the next curve to end, and repeat this as many times as you desire. When your happy with your curve, right-click and select 'Strole Path'. When the window comes up, select brush for your tool. Then hit ok. Right click again, and select 'Delete Path'.

Step 8. Duplicate your layer with your light beam/tentacle thing on it. Filter > Blur > Gaussian Blur @ about 5. Then merge (ctrl+e) your 2 tentacle layers to keep your layers orgranized.

Step 9. Repeat steps 7-9 with a smaller brush. Remeber to make a new layer for each tentacle. Be creative.

Step 10. Select your Elliptical Marque Tool, and draw a circle next to your light source. Then Edit > Stroke. Stroke them white @ 1 px. Then, grab your eraser tool, and delete parts of them to make it look like they are tentacle type things coming out of the glow.

Step 11. Duplicate that layer and Filter > Blur > Gaussian Blur @ around 2. Merge those 2 layers.

Step 12. Select your brush tool again and set the size to 1, and the flow to 100%. Make a new layer, and draw some particles around your brushing to make it look like it's emmiting particles. Duplicate, and Gaussian Blur @ 0.3. Make a new layer, and do this with a size 2 brush. Gaussian Blur this one a bit more. Continue doing this until you are happy with the results.

This is 1 of many ways I and others airbrush. Please note that I did not spend much time on this and if you really try, your brushing will most likely come out better than mine.

Hope this helped,
Overtone.