I was going to start off with an intro post, but thought I might save someone the time of building a bottle rocket antenna launcher when it won't help.

As mentioned above, cell phones in the US and Canada run at .8 GHz or higher, which is about 1000 times the frequency of AM radio. At these frequencies, reception is almost completely line of sight, which means that a higher antenna helps, but that's not the only thing that changes. Long wire antennas won't work, the loss in the wire is too high, and "long" at .8 GHz is measured in inches. Losses in transmission lines are very high at these frequencies, so much so that even a good external cell antenna connected with 10 feet of decent quality coax probably wouldn't work much, if any, better than the phone's built in antenna. Much better to move the whole phone up in the air somehow.

The article is pretty good. Cell phones do eat up their batteries trying to connect when they can't. They don't mention exactly why you should try turning around to get a good signal - it's because your head does a decent job of blocking the signal going through it. Holding the phone straight up in the air can help. Height is good, and being in the clear is good. Trees can reduce signal.

It's also probably good to know how your phone's GPS works, if it has GPS. With some, you can use the phone to display GPS coordinates even if you don't have cell service. The map won't work, but you should already have a map. By marking out a couple of points, you could use it as a really terrible compass, too.