These are the rules I follow when I am building power supply systems on my (small, mobile) robots:

• I always separate the motors' power supply from any other circuitry, to reduce interference (as @Manishearth mentions).
• For the rest of my electronic systems, I step down (using linear regulators) from a battery to each of the required voltages (usually 5V and 3.3V in my applications) in parallel.
• I always include standard diodes as the first component after the battery's connection to any circuit boards, to protect against damage by reverse polarity. The voltage drop here should be taken into account when looking at regulator drop-outs.

Most of my robots have been fairly simple and have not merited much in the way of redundancy, but a common way to add basic redundancy in the case of one power supply failing is through a simple relay-based switch box. This can be configured such that the power supply powers the relay coil in parallel to passing through the Normally-Open contacts to the robot. A second backup supply can be wired to pass through the Normally-Closed contacts. If the first supply fails, the relay will toggle over to the second supply. Cleverer systems can monitor power supply health and manually switch between primary and backup sources as required.

These are the rules I follow when I am building power supply systems on my (small, mobile) robots:

• I always separate the motors' power supply from any other circuitry, to reduce interference (as @Manishearth mentions).
• For the rest of my electronic systems, I step down (using linear regulators) from a battery to each of the required voltages (usually 5V and 3.3V in my applications) in parallel.
• I always include standard diodes as the first component after the battery's connection to any circuit boards, to protect against damage by reverse polarity. The voltage drop here should be taken into account when looking at regulator drop-outs.

Most of my robots have been fairly simple and have not merited much in the way of redundancy, but a common way to add basic redundancy in the case of one power supply failing is through a simple relay-based switch box. This can be configured such that the power supply powers the relay coil in parallel to passing through the Normally-Open contacts to the robot. A second backup supply can be wired to pass through the Normally-Closed contacts. If the first supply fails, the relay will toggle over to the second supply. Cleverer systems can monitor power supply health and manually switch between primary and backup sources as required.