The best method and really the only one I have seen work is an in building DAS(Distributed Antenna System). This method uses a Donor antenna on the roof and then a BDA (Bi-Directional Amplifier) in the building connected to a series of Antennas or leaky coax. The main problem with this is since you are installing an amplifier on a private radio frequency owed by the operator you are trying to connect to, you need permission and in most cases it will require them installing the system and most likely owning it as well with you paying a fee to lease it. You may look at offers from carriers called Femto Cells which in a sense are BDAs. Take a look. This may be a suitable option for you.