One of the biggest bangs for the buck in improving your cell signal is to attach an external antenna to your mobile device.
Antenna Adapter
This is a typical RF port on a cell phone:
Many cell phone do not have RF ports that are easy to get to as the one pictured. Also, your phone may have a port but no external antenna adapter has been developed for it. With some providers, these types of phones are getting harder and harder to find. If there is no adapter for your phone, you might consider changing phones or using a universal powered amplifier (see more info below).
This is an external antenna adapter required to connect an external antenna to your cell phone or cellular broadband card:
Adapter to Phone
One end connects to your cell phone and the other end screws onto an external antenna.
This is an external antenna:
Connect the three (cell phone, adapter, antenna) together to improve your signal quality.
Unfortunately not all phones or broadband cards have RF ports that enable you to connect an antenna to it.
Another option for these phones is the universal powered amplifier such as the Wilson SignalBoost or Wireless Amplifier.