If the interference is caused by other access points, there are materials and things you can do to limit the bleed-over from the other floors. Unfortunately, this tends to be both expensive and difficult to convince your building owner to allow.
Aside from that, you want to put your access points on the least used 802.11g channels (more on this in a bit). If it's really crowded, this may involve contacting those running the access points on the floors above and below and (politely) asking them to adjust which channels they use. When talking with your neighbors, keep in mind that you both have equal rights to the spectrum, but you don't have a lot of leverage in forcing them to make a change. This means you might even want to offer to help them if they lack the expertise to do it themselves. The only other card you have to play is that your equipment causes just as much interference to them as theirs does to you, but that's kind of a jerk way to address the situation.
As for evaluating channels, there are some things should know. First of all, get InSSIDer. It's free, and will show you exactly what access points are using what channels. This will help with planning. Note that InSSIDer only detects interference from other wifi access points. Other common points of interference include cordless phones, microwave ovens, and running water (I manage wifi deployment in college dorms, and showers are murder on signal because they reflect and scatter wifi signal, turning your own broadcasts into interference).
Additionally, know that even though there are 11 channels available, the channels bleed over. In practice, there are really only three channels you should use: 1, 5, and 11. There's less bleed as the signals get weaker, and so sometimes you can expand that to four channels if no access points with adjacent channels are placed near each other, but generally you want to stick to these three, especially when you already know there's enough density for an interference problem.
With only three real channels available you will need to put some thought into deployment. What you don't want to do is something where you negotiate one channel for the floor below, one channel for your floor, and the final channel for the floor above. This will work, but it's not optimal. Rather, you want to stagger the channels so the nearest access point is never on the same channel as it's neighbor, even if they happen to be on the same floor.
Finally, this means you'll need to turn off the part of your wireless controller that automatically adapts the channel each access point uses. Since your system doesn't also control your neighbors' access points and you're needing to play nice with them, you'll want to keep this static.