I unfortunately don't have much advice to give, but I do know exactly what you mean. Your description of "blank mind syndrome" sounds like me exactly. I can never think of anything to say when I'm with another person, though when I'm alone I have a pretty active mind. I've also done things like write down my phone number and address before making a call that I knew would ask for them, just to be sure I didn't stutter when they asked.
The only thing I've found to help a bit is to basically try to guess what the other person will ask and come up with lots of possible responses/questions beforehand. I pretty much imagine myself having whatever encounter I'm preparing for and imagine all the things they might say and how I'd respond until I've had dozen of imaginary conversations with them and will hopefully have some "path" to follow no matter how the conversation goes. This works OK for meetings at work and other structured interactions, not so well in a social setting though.
I have heard something that seems to help in more informal social settings. I've seen it stated several ways, but the one I remember is from "How to Win Friends and Influence People." To paraphrase, "You can make more friends in a week by being genuinely interested in other people than you can in a year by trying to make other people interested in you." I'm not sure how well it works in forming long-term friendships, but it does help to keep conversation flowing. Just simple, non-profound questions about the other person seem to work OK. It also keeps the other person talking so you don't have to.
As far as not remembering what you've come up with beforehand, maybe you're trying to remember too much? For normal social encounters, I usually try to come up with 1-2 questions beforehand that I can ask the other person so they're easy to remember (though this only works for me when I already sort of know the other person. I'm not very good at conversations with total strangers).
Well, anyway, not sure that was much help. I do know exactly how you feel, though. I hope someday I figure out how to have normal conversations without so much preparation.