My last few beers (all grain) have been a little astringent--there's a dry, almost chalky aspect to the beer. What are some things I should do to bring the astringency down?
Mineral profile of the water and pH are 2 of the leading causes of astringency. Are you experiencing it with a variety if styles or just one? It can be especially prevalent in dark beers if your pH is too low. Dark grains will lower your pH and you need to do something to get it back up into the 5.2-5.4 range. Contrary to the old myth that sparging over 170F will extract tannins, the real culprit is pH. I crush very fine, I usually sparge with near boiling water and my efficiency averages about 85%. All of those are things that have been cited as causing astringency, but I have no issues because I pay attention to the pH.
I would also add that the scum above the krausen line is very astringent, so don't mess around too much with your fermentor, since this very bitter, astringent stuff will get mixed into the beer. It's best left out.
I made this mistake once, when I was dry-hopping with bags, and wanted to get the beer and hops all stirred up to get the full effect from the dry-hop. But the beer came out astringent. It wasn't my mash, since I used extracts. It wasn't over dry-hopping either. The real difference was I rocked the fermentor a good amount for 4 days.