My understanding, which is limited, about cell phones and signal strength is that its effected by many factors including your location, tower location, interference (electrical, metal, etc) and weather. But I was told by a tech at Sprint that the best way to improve the signal strength on my phone was to turn it off, wait 1 to 2 minutes and then turn it back on. Not only does this allow the phone to "re-aquire" the network, it will also allows it to receive any network updates that might be available. And wouldn't you know it, everytime I've done this, my signal has always been stronger.
Oh, and make sure your antenna is up, many people forget this simple, but effective, way of improving the signal!
