Use the linear gradient with the standard black/white-colors and set the mode to difference. Now you can create contrasty forms by clicking and dragging the gradient-cursor in your document. You'll see how it works in the videos below.
The second tool I'm using is the simple brush-tool with a softened brush and a size of about 50px. You don't have to paint abstracts - just create some simple lines or points with the brush-tool.
|At last I'm using the liquify-tool (Photoshop CS and higher) to get some interesting forms. The liquify-effect is applied to the brush-layer which still contains simple brush-lines.|
These techniques are used in the videos below. The result is a good basis for abstract backgrounds and graphics.
||Just start with dragging some gradients until you're satisfied with the result. Then paint a simple shape with the softened brush and liquify it.
After that duplicate the liquified layer and erase some parts with a softened eraser as you can see it in the video. That's it for the first abstract!
||This is another variation which uses the same techniques as above. There's one more special shortcut I'm using which is CTRL-SHIFT-ALT-T - this shortcut relatively repeats and applies the last made transformation to the current layer (the layer is duplicated with the applied transformation). So you're able to create an interesting effect.|
||In this example I'm adding more points with the brush-tool so I'm getting more details in my liquified layer.|
As you can see you can do a lot with the basic tools of Photoshop. Just play around with the settings and be creative with the use of transformations.