VZW Network Extender
http://www.thinkgeek.com/gadgets/cellphone/9823/ »
•The Dual-Band model works with all carriers listed above. Cingular (AT&T), Verizon, Alltel, T-Mobile, Metro PCS, Cricket, Suncom, and Sprint (not Nextel).
It is a bit pricey and I do not know of anyone who has used one but I would say it is worth researching.
http://www.thinkgeek.com/electronics/cell-phone/b32e/ »
#1 - You have to have good coverage outside your apartment.
#2 - You probably need to have landlord permission to install an antenna on the outside of your apartment. If you have a patio a thru glass antenna may do the trick if coverage right there is strong enough.
Yagi antennas work best and will work for most every carrier because they are frequency specific, not technology specific. You would need an antenna that works on both 850 Mhz and 1900 Mhz.
Good luck.