Usually circuits with less than 24 volts are considered safe. There are a few exceptions to this rule. One of the worst shocks I have ever gotten was across a circuit that had a 20 volt 180kHz square wave. I touched a pair of sharp solder joints and the metal penetrated my skin, making a very good contact. I wasn't being careful enough because I thought that 20 volts couldn't hurt me! I felt the after-effects of this shock for about three days.

These rules will usually keep you out of trouble. The most important rule is to not goof around with high voltage, and be careful.

Rule 1 is important because the idea is that you do not want any current from an electric shock to go through your heart. Your heartbeat is controlled by a little electronic circuit near your heart, so you don't want to blow it out with too much voltage.

This list comes from the ARRL Handbook, published by the American Radio and Relay League. I added a few rules of my own.

1. Keep one hand in your pocket when working on "live" circuits or checking to see that capacitors are discharged.
2. Include a conveniently located ground-fault-interrupt/circuit breaker in the workbench wiring.
3. Use grounded plugs and recepticles on all equipment that is not double insulated.
4. Use double-insulated equipment when working:
   • outdoors.
   • on a concrete or dirt floor.
   • near standing water.
5. Use an isolation transformer when working on ac/dc devices.
6. Switch off the power, ground the positive lead from the supply and discharge capacitors when making circuit changes.
7. Do not subject electrolytic capacitors to:
   • excessive voltage.
   • ac.
   • reverse voltage.
8. Test leads should be well insulated.
9. Do not work alone!
10. Wear safety glasses for protection against sparks and metal fragments.
11. Always use a safety belt when working above ground level.
12. Wear insulating shoes such as relatively non-worn-out sneakers.
13. Wear shoes with nonslip soles that will support your feet while climbing.
14. Wear a hard hat when someone is working above you.
15. Be careful with tools that may cause short circuits.
16. Replace fuses only with those having proper ratings.
17. Know where the nearest fire extinguisher is.
18. Encourage your friends to learn CPR.