What are some ways I could interest my friends, girlfriend, parents, etc. to learn and use Linux? It's usually not feasible for them to let me install Linux on their computers, even for a dual boot. Are there other things to get them hooked?
A good way to demonstrate Linux features and for others to play around with, is to boot off a live CD for your Linux distribution. That way no one has to worry about partitioning any hard drive or installing any (corrupted,evil) software until they eventually choose to go with it. The fact that it's for free to try out should make the choise easy. Make a couple of live CDs and distribute them. I suggest Debian or Fedora just show the basic features.
I just put it in front of them and let them use it. I find that many people, while they do not hate it, are fed up with Windows OS. Some are put off with the price of a Mac. Let them experience a good windowing system, Gnome, KDE, Xfce, see how responsive it is. I used to have dual boot just to use Quicken. Many years ago I moved to GnuCash, haven't wanted to go back. There are plenty of apps on both systems.
Really hard question. First of all you need to understand that you need to support this people after you interest them. From your list parents is easiest targets but you must to think about every single user as about single situation. Usually friends plays windows games, so you must show some good linux or crossplatform games and lie that they works much better there. If they just use browser you can lie that without antivirus their PC will work 50% faster and compose something else. For parents you can compose everything, maybe windows is an UFO system and they must use Linux to not afraid aliens.
Yes, you always must lie here, in my opinion. I have good experience with linux and use it often. even I know some great linux distribution maintainers and developers (I will not say names). BUT no one of them even me use linux as desktop. I don't want to discuss why not, it's really holy-war, but everyone lies. That must be much harder for you to find some real arguments for newcomers if even people who love and work on linux can't find them for themselves.