This is my take on drawing pixie dust in photoshop.
First of all create a new document of about 1000px by 1000px 300dpi, this will be to create your brush, ultimately you can experiment with different brushes but this will give you a good starting point of how to go about starting it.
now using some of the custom shapes that come with photoshop cs3, draw some stars of different sizes on the blank canvas. You can load different shape and also create and load your own, but for now lets keep it simple and use the ones that come as standard.
Try not to overlap the stars but make sure you add in some little cute ones like below
now on each layer add a layer effect of outer glow to each star, you can play with the setting but I found the below setting got a fairly good result
Now to create the brush, Select all the canvas and copy the lot, then go upto the Edit menu and Define Brush Preset
Give the new brush an obvious name of your choice, I called mine Pixiebrush!!
Now the brush will appear in your brushes pallette and you can adjust the setting to acheive the pixie dust effect. I found that a little tweak of each setting would get the results I wanted, there are no set parameters to follow here and I would say just play with them until you are happy, below are the settings I used to get my results.
And the final results can be seen below – hope this is useful and enjoy.