My 1999 and 1964 half dollars look the same to me. I dont do a lot of "Remember the date" tricks. But go to the bank and buy buy buy.
If you bought every half dollar in the bank, you would not get a single 1964 silver half dollar.
The best advise is this:
If you don't do tricks where it matters if the coins "talk" (i.e. make noise when rubbed together) then the silver 1964 Kennedy half is a very pretty (and visible) coin, which will usually still have nice milling left on the edges.
If you are on a tight budget, go to Jamie Schoolcraft's site and order just the '64 Kennedy "expanded" gaff.
Once the gaff arrives in the mail, take it to a local coin shop and ask for "junk silver '64 Kennedy's". Usually they will dig up a coffee can with anywhere from a dozen to twenty or so coins. Go through the coins one at a time and see if they fit your gaff. (Be VERY careful and don't force anything or you may end up in a 'stuck' situation!) Once you have a group of coins which 'fit' well, then start looking at the edges and faces to look for obvious nicks, scratches, etc. Buy at least four and a couple extra if you can afford it. (By the way, they will probably think you are insane... This is normal...)
Take them home and clean them up. Silver polish probably isn't necessary (and could possibly be toxic considering you are going to be handling these a lot!) Toothpaste usually works fine. I'm sure other people use other things to clean coins, but read the ingredients carefully! Many silver cleaners can cause cancer! Since I perform tricks where coins appear to come from my mouth, I'll stick with the tooth paste.
Hope this helps,
-ThisOneGoesTo11