External antennas work great for me.  I bought mine from
www.alternativewireless.com .  I believe Cingular sells them as well.
Actually I use my external antenna while I am on a boat; I haven't
really needed it in a car except maybe on occasional road trips.
Wilson and Larson are two well-known brands.  Check out the website
above for more info.

wilson 12" dual band magnetic mount works great with my audiovox smt
5600,  i got it from www.accessorygeeks.com 

only hitch, i had to file down to the outer socket of the adapter to
allow the inner male connector to make good contact with the phone
socket.