Re: Keeping vintage 6-point trems in tune
The main thing is to have realistic expectations. If you don't use them as they were designed to be used, they won't stay in tune as well as they are intended to. They aren't designed to bend notes very far off pitch, or to provide dramatic special effects. They are designed to give you a mild vibrato, of the sort that is generally achieved with the left hand by rocking your fretting finger back and forth. They also work much better if you alternate your vibrato between pulling up and neutral, as opposed to pushing down and neutral (which is how almost everyone uses them).
That said, a proper setup, and especially a properly shaped and slotted nut, will help. Most Fender guitars are improperly set up when they leave the factory, and are often even worse by the time you get them from the retailer. Gibson is even worse these days. For at least ten years now, I haven't played a brand new standard production Gibson or Fender that did not need a setup, a new nut, and fret work to be what I consider well set up. (The few exceptions were Japanese models, which seem to be set up much better from the factories, in general.)
It will also help you to minimize the number of wraps around the tuner posts, to wrap the strings in a self-locking manner, and to try your best to equalize the break angles of all the strings over the nut. That means wrapping the low E, and usually the A as well, up the post instead of down.
Locking tuners make it very easy to minimize the number of wraps around the tuner posts, and they also often come staggered, which will help you equalize the break angles over the nut. But you don't need locking tuners in order to minimize wraps. I wrap under one turn, even with standard tuners. Once I started doing that, my Strats (and all models, really) started staying in tune much, much better.
Lubrication helps too. I prefer to replace the nut with a Tusq XL nut, personally. (It must be the XL version; plain Tusq is no more beneficial to lubrication than any other plastic nut.) But I have also used graphite lock lubricant and Neo Lube No. 2 to great effect. They ae both basically the same thing, but Neo Lube is better, because the graphite is carried in alcohol, which evaporates. The graphite in the lock lubricant is carried in machine oil, so it runs all over the darned place, and takes quite some time to dry. Pencil lead is often suggested, however it is not pure graphite, but graphite powder in a clay binder. It tends to cake up, which can actually cause more binding than when stock. Neo Lube is by far the best lubrication product I've found for guitars. Here is the first Google hit about it:
http://www.micromark.com/neolube-2-fl-oz,8383.html. And, most importantly, don't think that lubrication is a way around having a properly cut and slotted nut; it isn't. Even Tusq XL, graphite, and lubricated nuts need to be slotted right in order to perform their best.