Toxic pretty much boils down all the basic ways people get in...
however just to add...
WEP cracking software is alot smarter these days.
About 5 years ago, it could take a week (about 3 days real work time) for the "average hacker" with a basic knowledge of Linux to hack a wireless network feeding to about 10 wireless bound computers. Of course, the more computers that are on this network, the faster it gets done.
Today, there are pieces of software available, to anyone that is willing to read and learn a little, that enables people with a good understanding of wireless technology the ability to "hack" a wireless home network in about a 2 days time (some faster). Varies of course on only a laptop or computer being on all the time and a wireless device being on all the time. But this is only 64-128 WEP that we're talking about.
This is a big jump considering 5 years ago, in the above scenario, it would take 1 person over a weeks time to be able to crack a WEP Key with 1 laptop online and 1 wireless device.
But....times have changed. In about 5 years, we are seeing the average person having nearly double the wireless technology in their home than before. This, of course, includes more wi-fi adapters, routers, APs, Signal Boosters and Bridges.
WPA PSK are a lot more secure and there are tools on the web that help you generate keys rather than coming up with a password or phrase to generate one for you.