Here are some basic facts about event power that will help you avoid these pitfalls.
Plan in Advance
Power should not be an afterthought. Whether you are getting power from an existing power distribution point, from a generator, or from temporary power outlets (which can be setup by the utility company), you absolutely must consider a few things well in advance of the event: 1. where you need power, 2. how much power you need, and 3. what quality of power do you need. Lets talk about each of these....
Where Do You Need Power?
The first thing to consider is where you need power. You'll want to consider this early on, especially for a large event, because you don't want to have to redo your site plan, and you definitely don't want to be running to home depot the day of the event to buy orange extension cables, and, if you need more than 20 Amps of power somewhere where you hadn't counted on it, you may be out of luck. If you need a large amount of power, you'll need big extension cables. These cost real money to rent and can take time to track down. If you are planning a large event, you'll probably want to create the site plan to minimize the number of locations you'll need power. If you are planning a small event, such as a small wedding, you may still want to make special considerations for power -- you don't want guests tripping over an ugly extension cord because the band setup on the other side of the room from the only pair of dedicated outlets (more on dedicated power in the quality section).
How Much Power?
Most sound companies and other vendors pad the amount of current they will draw (measured in amperes, or just amps). For example, they may request 30 Amps, when their equipment only requires 15. However, they are not just doing this "to be on the safe side": many pieces of equipment draw much more power when they are first switched on, and often this is not reflected in the equipment's ratings. If you are dealing with separate vendors, make sure they each get a circuit of at least the size they requested. If you need to use extension cords, keep in mind that the ratings of the extensions should exceed the requirements -- especially if the cables are long, because long cables themselves can cause a significant drain on the power system.
What Quality of Power Do You Need?
Here's the biggy: not all power is equal. Audio and video equipment in particular is very susceptible to fluctuations in power, "noise" on the line from other equipment (especially anything with motors or dimmers), and many other things that most equipment won't even notice. Audio/Video people will typically refuse to share a circuit with anyone and many won't share a generator. Considering the cost and sensitivity of the equipment to poor quality power, this is not unreasonable and should be expected.
There are other issues. Vendors requiring large amounts of power typically need something other than your run of the mill 120 volt / 20 Amp power. Connection types, current requirements, phase and power factor are just some of the issues you may have to wrestle with. Make sure to get it right or there may be no event.
Power safety is obviously very important. I can't begin to cover this topic in a little post here, but for starters, make sure to keep it away from water, children and anyone else who doesn't need to get to it. Rented generators should always be properly grounded as required by local code. If cables runs have to me made across an area where you expect pedestrian traffic, be sure to mark it carefully so that no one trips. If you are unsure about any safety issue, contact a licensed electrician.
Of course, power is complex, and there's more to it than that. Generators make fumes and need to be refueled. Temporary power tends to be ugly, expensive and may require additional permits and inspections, and, since many people don't understand power well enough to tell you their requirements, you might end up unable to connect some equipment as expected. Be sure to exchange information carefully, and, just in case there's a problem day-of, make sure you have an electrician available.