Improving Cell Phone Reception In Buildings?

Amphigory asks: "Not too long ago, I got a digital cell phone. Unfortunately, this phone simply doesn't work in my office even though it works in other parts of the building and even outside the building. For a variety of reasons, I would really like it to. What I'd like to be able to do is put some kind of antenna or maybe a retransmitter from my office to the roof of the building that would allow the signal to get through. Has anybody out there ever done anything like this?" I've heard of systems like this for large convention centers, but I'm not aware of the details. Can anyone shed some light on this subject?