I work with an isp that provides 802.11b wireless access for some of its customers.
A couple of thoughts.
1. First do so calculation to determine the height needed for your antenna. here is an article that will get you started
http://www.oreillynet.com/cs/weblog/view/wlg/197
basically, if you can see the location you are trying to reach, either visually or with binoculars, then you should be able to reach them.
2. locate the access point as close to the antenna as possible.
the longer the cable run, the more the line loss. Antenna cable is expensive, ethernet cable is cheap. Alot of access points make a power injector that allow you to run the power throught the ethernet cable, saving you the hassle of pulling power to the access point. Dont try and make your antenna cables, they are not RG6, they are alot thicker w/ different ends. Hyperlink is a good place to get your wireless stuff from.
3. You may have to put an amplfier on at least the omin-directional antenna. you can legally run up to 4 watts of
an omin-directional antenna. Boost is asymetrical though, boosting mainly the transmit side. Limit the allowed speeds on the access point as well. if you internet connection is a T-1, set the access point to a maximum allow connect speed of 2Mb
4. of course wep and mac address security are necessities