My T-Mobile Web'n'Walk III (AKA Huawei E170) 3G dongle has very variable reception while I'm at home and has no external aerial connector. Are there any tricks that actually work to improve reception without involving the operator installing an in-building cell?
An antenna could help with this. This link could help you pick out which one is right for you.
You could try a cellphone signal booster. Here is an article and installation tutorial on ThinkGeek.com:
This Dual Band Cellphone Signalboost Extender has a frequency range of 824-894 MHz / 1850-1990 MHz and boosts phone coverage from any cellphone provider in the USA except for Iden and Nextel. Plus in addition to enhancing signal-strength on voice calls it works with high-speed data transfers using EDGE, 3G, and EVDO.
Important Note: You will need to have some signal (at least one bar), for the Cellphone Signal Extender to work. You can't extend what's not there... right?