Go ahead - I'll wait.
Not you, of course. Like me, you should only use your Powers for good.
Both these guys use Linksys as an example, but it really doesn't matter which brand wireless router you have: Netgear and D-Link will be the same.
So how do you keep this from happening to you? It's actually pretty easy.
Step 1: Change the router's password. Both of these demos start by assuming that the router is a Linksys, and then using the password that ships from the Linksys factory. If you change this password, then the Bad Guys have to guess the new password, and you've made things enormously more difficult for them. Just choose a good password. On a Linksys, you do this via the Administration menu.
That's it. The YouTube videos won't work on you now. Well done, you.