1. Check your Tools: Make sure your gadgets have insulated grips and ground fault circuit interrupters. If you're using extension cords, choose ones with the heaviest gauge specifically designed for the outdoors.
2. Invest in Safety Gadgets: Hospital bills aren't fun. Invest in preventative measures such as leather gloves, hardhat lined with flame resistant material, and safety goggles with shields. Might seem a bit much for now until sparks start flying towards your face.
3. Wear Appropriate Clothes: Wear a flame-resistant long sleeved shirt when working with electric. No nylon. No synthetic stuff. That stuff will melt and isn't fun when it gets stuck on your skin. Leather boots help too.
5. Never Work with a Live Wire: Here's a rule of thumb that applies to anything electrical - always make sure wires are not live when working on it. Live = electricity running through it. Shut off breakers and test everything before you start digging in. Hand-held voltage testers are cheap and will go a long way.
6. Check Local Outdoor Wiring Codes: Know your local safety codes. This will make it easier to buy the right plugs, wires, electrical outlets, lighting fixtures, or appliances before getting yourself subpoenaed by city authorities.
7. Know Proper Wiring Techniques: While doing it yourself will save money, if you have no idea how to fix or install wirework, hire a professional. Loose connections, improper installation, and other issues may lead to fires and shorts, which are like, totally not cool when you're throwing a patio party.
For more details, you can check out the standard specific guidelines over at eHow.com.