Buying a dehumidifier won't do you any good if you don't know where to put it or how to keep it running well. Proper maintenance will ensure it works longer and a lot more efficiently.
Most dehumidifiers have air-discharge units mounted on the top. These can be placed almost anywhere without much worry. If air discharges from the side, though, the dehumidifier shouldn't be placed against a wall.
Where to locate it
If your home is heated by forced air and you have central cooling, a fan likely moves air around continuously. In this case, it's not as important where you place the dehumidifier because it should be removing about the same amount of moisture wherever it sits.
For convenience, many people choose the laundry room because of easy access to drains. Laundry rooms are also preferable because of their warmth; dehumidifiers work more efficiently in warm air.
If air isn't constantly circulating, a dehumidifier will work best where dampness is the biggest problem. If the area is cold, only a dehumidifier rated for temperatures low enough to function in that space should be used. Bedrooms aren't generally a popular choice, since noise is an issue with many dehumidifiers. And without a drain, emptying can be a hassle. Warmer air near the unit is a side effect of running dehumidifiers, so if that's a problem, you won't want to put it where you'll be constantly exposed to it.
If the dehumidifier is for a basement, it should be easy to find a drain to direct the hose into. Finding a dehumidifier that runs better in lower temperatures is a good idea if your basement is cooler than most.
Let your dehumidifier live up to its potential
To maximize the dehumidifier's efficiency, keep it in a room the size it's rated for and close doors and windows to allow it to do its job. It should be located away from sources of dust and dirt, which can clog grills and coils. Carefully clean (with a small non-wire-bristle brush) and vacuum the coils each season.
Internal filters should be cleaned about every two weeks, less often if you find they aren't picking up as much dirt. And while certain models are somewhat self-regulating, they can't predict weather changes. Keep a hygrometer on hand and reset the unit accordingly. Frequently emptying and cleaning the water bucket is also essential to prevent mold and mildew growth.
You should always at least follow the manufacturer's recommendations, but a little extra care might save you time and money in the future.