If you’ve signed up on a 12/18/24 month plan and are getting bad signal at home, you can postpone your panic attack a little by considering some of the options you have. 

Many of the USB keys and dongles out there have the possibility of connecting an external antenna, and people mention a pretty nice jump in strength and speed when using such an antenna. (example) It might be worth it, although the cool factor is lowered a bit.

If you're using a HUAWEI 220 you have no apparent possibility to attach an external antenna. But actually the modem contains a hidden connector so it is possible. Do-It-Yourself users should read about this here. 

Also, you might want to consider buying a long USB cable. It's a lot cheaper than an external antenna. An USB cable will make it possible for you to place your mobile broadband modem in or near your window etc.

Alternatively, use a beer can… ;-)