I'm apparently mineral spirits challenged

Well first off what kind of poly did you use, oil based or water based.  If its water based you would actually want to use soap and water.  If its oil based, I typically rinse with mineral spirits, then use the hose to blast off the excess and to avoid wasting mineral spirits.  Then I repeat several times.  You can still use the brush as long as its not completely stiff.  Typically I just bend the bristles if its a little crunchy and the brush is fine.

Sorry, I should have specified, I guess!  Totally oil-based.  The oil-based world is all new to me ;)  (and I'm petrified of it...I'm soooo overly cautious freaky-deaky about used rags)

TNSLB:

Here's my technique, and I use mostly oil-based stains and varnishes. The brush will never regain that silky brand-new feeling, but mine don't get crunchy. there are a lot of steps, but not much actual work.

TIP: I start painting by dampening the brush with the appropriate solvent. It helps keep the base of the brush clean.

1. wipe off excess into can
2. wipe off even more with paper towels or by painting onto newspaper - get it as dry as you can
3. put an inch of mineral spirits in a jar that's just big enough to hold the brush
4. set brush in and wrap the top of the jar tightly with aluminum foil to keep it from evaporating
5. let it soak up the mineral spirits
6. agitate it occasionally for a few hours
7. squeeze the mineral spirits out with a paper towel ... squeeze really hard, blot, whatever
8. Now wash it well with soap and hot water to get the final bits out of the brush:
9. Squirt dishwashing detergent or shampoo or hand soap on the brush and work it in well.
   I use a fingernail brush and scrub from the base out to the tip to make sure it's well worked in
10. add a bit of water and wash it some more
11. Rinse very thoroughly, making sure you run water into the middle hollow of the brush
12. Blot it dry and shake it to make sure the water is out of the base of the bristles. (this is messy, so do it outside)
13. Spray the bristles with a bit of leave-in hair conditioner, work it in well, and smooth out the bristles.
14. Wrap the brush in a bit of paper to keep the bristles together for storage

DISPOSING OF THE RAGS AND PAPER TOWELS: Yes, they can catch on fire if you pile enough of them up. If you have a large number of them, like when you are doing a whole floor, drop them into a bucket of soapy water (laundry detergent) until you have time to dispose of them. They will not catch fire when they are under water.

I lay them flat on a railing or sawhorse until they are stiff and dry, then toss them in the trash. When the liquid is gone - solidified or evaporated - they can't catch fire.

Painting Tips:

• When you paint, don't dip the brush deep into the can. Just use the bottom 1/4 or less of the brush and dip more often.
• If you see that the bristles are getting clogged up, stop and rinse it with the appropriate solvent, blot it dry, then continue