As Tester101 says, extension cords are not meant for permanent wiring. If you really need an outlet in a given location, you should consider having one installed.
But there are a number of things to keep in mind when you are using extension cords.
- Don't staple them in place (big risk of damage)
- Don't use a cord that is rated for less amperage/wattage than you are using
- Don't try to use a two wire/two prong cord for a three prong device
- Don't octopus the cord (adding a multitap adapter at the end of the cord)
- Avoid using extension cords for any high draw devices, such as refrigerators, toaster ovens, irons, hair dryers, etc.
- Don't leave exterior cords plugged in
In general, the use of three prong, heavier (wire gauge, not weight), shorter cords are preferable.
And make sure they are out of the way and not a trip hazard (no crossing thresholds, other footpaths).