I am in a condo building with 14 units. Up until a few months ago I had no problems connecting with my wireless network. Then I started having dropped connections or failure to connect on bootup. I downloaded and installed a program from Macwireless.com, called Airport Radar, which displays all available networks in the building as well as their channels. I found that there were a LOT of wireless networks in this building. Seems that there are 11 networks in these 14 units. Yikes! No WONDER I was having problems! With the ease of setting up home wireless networks these days, just about everybody has one.....
First thing I did was to change the channel from the default channel 11 (someone else already on there) to an unused channel. That helped some. Then, most importantly, I added an Airport Express to my already-existing Airport Extreme and that solved the problem. The Airport Extreme is in a bedroom, since it's connected to the cable modem, and then the Airport Express is plugged into an outlet in the living room, which extends and strengthens my network signal. No more connectivity issues!