Map out neighboring wireless networks. This is more a channel issue. If neighbors are on 1 or 6, you want to be on 11 to avoid contention. If you need multiple APs, you want to stitch your channels for the least co-channel interference. You don't want two adjacent AP on channel 11, for example. Also, look for noisy appliances in the area like 2.4 or 5 Mhz phones, microwave ovens, and other devices that radiate in the vicinity.
Explore your options for access point placement. Putting access points higher up -- either on or towards the ceiling -- significantly helps coverage. If you have a large or complicated space (lots of walls, metal, bookshelves, etc.) you may need multiple access points to provide good signal to just a few users. You want to place APs where you can practically get wiring runs to them, and most are designed for optimum coverage at ceiling height. That being said, sometimes another AP in an easy-to-reach location compensates for mounting location trade-offs. Don't simply string Cat 5 cable through drop ceilings. You may need plenum rated cable. Check your local codes before stringing cable.
Determine the 802.11 protocols do you want to support. 802.11g is still the most prevalent client radio out there. To future-proof, buy 11n. It's backwards compatible with 11g and 11a, and gives a better overall cell quality.
Laying out wireless networks is part science and part art. You have to make a judgment call about signal strength and user to AP ratio. You have to take into account access point placement based on network and power availability. Finally, you can do all the planning only to find construction materials are blocking good signals or a neighbor turned up an access point next to you space at full power. With some careful planning, working with your neighbors, and on-going monitoring, you can have a good wireless experience.