positive and negative voltages from power supply
You can make a rail splitter like this: -



It (more or less) actively "fights" to keep the output terminal (VGND) half way between the upper and lower power rails. The problem with just using resistors is that they obey ohms law and, if you start to draw current, the voltage shifts towards one rail or the other depending on the direction of current flow.

You can also gets chips to do this such as the BUF634: -



It works with power rails from a couple of volts to +/-18 volts and can deliver 250 mA BUT be aware of power restrictions and heatsinking.

For lower power applications, a simple op-amp with do the job.

Depending on what current draw you want try this google search for circuit images.