Up-front analysis is key. Have a scale drawing of your area so you can map out your AP placement. A free tool like Meraki's WiFi Stumbler will help any environment size up neighboring wireless networks that need to be worked around by showing adjacent network names, channels in use and how strong they are being felt in your space. Once you map the airspace, you can design your own access point placement. Remember that your airspace will change over time as neighbors add or remove their own access points, but you can adjust your network as needed.
Estimate how many users are going to be on the wireless network at a given time. If more than 15-20 users might be on the wireless network at the same time, you'll need to plan for more access points even if just one can fill the given area with signal. Wireless is a shared medium and when you get over 20 simultaneous users, contention will become a limiting factor in performance.
Determine the minimum throughput you'd like to provide. 802.11 a/g offers upto 54Mbps while 802.11n can reach, in theory 600 Mbps (though actually data rates will be much, much lower). The data rates will increase and decrease based on conditions in the airwaves. Balancing the data rate and number of users is a tricky process. Web and mail traffic might be considered lightweight, but a single YouTube video is usually around 500 Kbps. Consider what your users are likely to be doing; 10 users all streaming video might max out an 11g access point, for example.
Map out the size of the area do you need to cover. APs are usually "rated" for open areas when distances are claimed. You might get 200 feet of good signal outdoors, but only a couple of rooms inside. Construction materials make effective wireless barriers, particularly in older buildings where metal screening was often used to hold plaster. Verification is a must. If you see signal falling below 70 dBm, you need another AP. You don't need to have your AP blasting at full power either. If you are at an edge, try adjusting the power levels so that you have adequate power for your area, but are also being a nice neighbor. It's also a good time to go talk to your neighbors who are using maximum power to see if they can turn it down a bit.