A lot of people ask me what kind of brushes I use and how I made them. Usually when I paint I only use a few brushes, most of which are just Photoshop presets with a few tweaks to the brush settings. When painting the figure I normally use the standard brushes and save my custom brushes for textures and for backgrounds. These are the brushes I use, although a majority of all my illustrations are painted using primarily the two brushes circled. I find that most of the time there are too many brushes to choose from, and most of them are just plain gimmicky. I like to stick with just a few of my favorite brushes when painting. This also helps to improve your speed, since you do not have to look around and try and decide which brush to use next. It also allows you to learn how to paint different textures using the same brushes, instead of relying on a brush to paint a texture for you.
Let's start making a custom brush by putting down some random shapes and lines. Try not to make anything too symmetrical, as this will make your brush look weird and fake. Sometimes what I like to do is use a custom brush that I have already made as a starting point. I lay that down and then start drawing and erasing on top of it. Now that I am happy with my shapes I am going to open a photo to use as a texture overlay. It doesn't really matter what kind of photo you use. In this case I used a photo of my sister's dogs just to show that you can use any subject matter and still get a cool looking brush.
Once you have a photo, convert it into grayscale and then go to Image>Adjustments>Levels (or ctrl+l). Drag the sliders around until you get some high contrast within the photo and then click ok.
Copy and paste the photo into the document with the brush we started making. On the layer with the photo, change the layer properties to something that looks good. In this case I used Overlay.
Now you can go back and add another layer on top of the photo and fix any parts that may cause problems or repetition. I painted out some of the lines in the top left of the picture.
To make this into a brush, go to Edit>Define Brush Preset. I am using CS2, so the wording may be different in other versions, but basically anything that says "Define brush" will work.
This brush will now be at the end of your brush list on the brush drop down menu. Select your new brush and give it a try. Right now the brush will look really ugly and repetitive. We are going to fix that.