People! You're crazy!
I'm an "Industrial Coatings Expert" (read "painter").
Red, shame on you! Leaving your brush in the poly all day everyday. And Surreal, do you
eat food out of that freezer?
Keep in mind that my advice is based on keeping my oil brushes in service for several different applications and long periods of time.
As Knothead said, use a good natural bristle brush for poly. The white chinese bristle brushes are better than the black chinese bristle brushes for polyurethane. They are easier to cut in with and will usually leave less brush marks than the black chinese bristle brushes.
You should clean your brushes when you are done with them. I don't let my brushes soak in thinner, unless they're already beat. They are usually epoxy resin locked into the ferrule, and letting the brush soak in thinner can weaken the bond. This is especially true with stronger thinners. I usually paint my brush out and rinse it in thinner. A half inch of thinner in the bottom of a 1 gallon can or rollerpan will do nicely. You'll need to rinse it in fresh thinner at least three times. Polyurethane is very sticky, so you will be able to tell when the brush is getting clean. Dip the brush in, squeeze the thinner out, repeat...
Natural bristle brushes should never be put in water. The bristles get all screwed up and stick together. It is very hard to paint with latex paint and a natural bristle brush.
Turpentine, mineral spirits, and paint thinner are all generally the same thing. I prefer paint thinner because it usually has a higher oil content and will keep your brush soft and supple for cutting in longer. Strong thinners will dry out the bristles and cause the brush to fray, whether you keep it in a shape retaining container or not. If you plan on keeping the brush in storage for a while, rinse it out in kerosene. Kerosene doesn't have the excessive oil content that diesel has, but will keep your bristles in a happy, workable state while they rest in your shop.
If you are using a synthetic bristle brush, just clean it with thinner. The bristles won't dry out in a nylon/polyester brush, but the same rules of soaking them in thinner apply.
I strongly suggest using a natural bristle for all oil based paints, except for epoxy. Epoxy paint doesn't matter; whatever brush you put into it is going to die a dry, painful death.
If you only want one good brush, I suggest a Chinex/polyester brush. I have a few that I use for only latex paint, but some of my co-workers have severely abused them and they hold their shape. They are excellent for cutting in with high quality paint, as the bristles are a little stiffer than some other premium brushes.
I'm sorry if I told you too much
(you can generally procrastinate a month before you get back to your project!) 
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