1. Use the same SSID (wireless network name) and password on each access point.  Your wireless clients will automatically connect to the strongest AP and will (eventually) switch over to the other ones as you roam around.  (If your wireless clients can still get a reasonable signal – say two bars or more – they will tend to stick with the same access point though).
2. Allocate each access point a different channel.  This is the key to reducing interference between the access points and thus allowing maximum throughput and range coverage for each one.
3. Only use channels 1, 6, and 11 as all the others overlap to some extent.  If you have more than three access points, re-use these three channels keeping same-channel access points as far apart as possible.

As an example - if you have three wireless access points, each should have identical settings for SSID (network name) and password, and each should be using a different channel: 1, 6, and 11.