How do I make the beef in my beef stroganoff more tender? Tonight I prepared some beef stroganoff with a recipe from allrecipes.com as a rough guide.  It came out really well, except that the meat was very tough.

I bought the meat pre-cut as "stew meat," as recommended by the guy at the meat counter at the supermarket, and thus not specifically chuck roast. I bought it last weekend, expecting to prepare it that day, but plans changed, so I didn't prepare it until now.

I covered the meat (~1 lbs, cut up) with some white flour, then browned it until it was maybe 70% brown (a little red still visible), prepared the rest of the sauce, then let the sauce and meat simmer for an hour.

Did I brown the meat too fast?  Did letting it sit in the fridge for a couple days make it more tough?
Supermarket 'stew beef' is notoriously unreliable.  Its often just scraps of beef that the butcher or market can't sell otherwise.  My stews were hit and miss for years while I tried to tweak cooking time and such. But then one day I watched a movie on rouxbe.com (paywall, sorry) and they talked about not using 'stew beef' - its often too lean and doesn't have the 'stuff' to produce tender, but juicy bits.  They strongly recommended cutting up your own from a chuck....and they were so, so, so right.

In short, cutting up your own chuck is by far the better route - if you're looking for 'stew beef'.