Decide that you will not be invaded. Letting yourself get mad and deciding to set a boundary is often all you need to do. Repeat this decision several times a day by peppering your life with little notes to remind yourself.
Say something. Practice saying something in advance (and practice it out loud): "I'm uncomfortable right now with what you're saying (or doing)." To people who touch you inappropriately: "I'm uncomfortable with you _______________. I'd like to ask you not to ________________." And step back. To cope with people who comment on your personal life when they have no right, reply: "Excuse me! How inappropriate!"
Empower yourself with language. Don't say "I can't" when you mean "I won't." Invasive people take "I can't" as an invitation to solve your problem ... so that you "can."
Take care of your inner child. Consider what you would say to your outer child about boundaries. You wouldn't say, "We wouldn't want your friends to be mad at you, so you can go to the slumber party and stay up until midnight even though you're only 5." No. You'd say, "It's my job to take of you. You can't go, but you will still have friends." Say to your inner child, "It'll be okay. I'm going to take care of you. You didn't learn this, but I can teach you."
When you notice boundary paralysis setting in, touch yourself. Even putting your thumb and forefinger together really helps; the physical connection is very grounding. This is because boundary paralysis is part of dissociating and touch is a way out of dissociation.
Practice your boundary-setting skills. You may not get it right at first -- keep trying. Here's a tip: Practice boundary-setting in low-key relationships first: strangers, sales clerks, waiters, and so on.
Be prepared for inner backlash. As Dr. Fredrickson says, there's a very predictable chain of reactions that occurs when you start setting limits: "You feel terrified beforehand, wonderful while you're doing it, and horribly guilty afterward." But remember, just because you feel guilty doesn't mean you're wrong.