Transitioning to Dominion late-game
One of the first things I try and figure out when evaluating a set of kingdom cards is to try and figure out which way I think the game will end.  Is it going to be a three-stack game or a province/colony game?  Are there cards that push it in one direction or another?  Am I going to try and push it one way or another, and what do I know about the tendencies of my fellow gamers (and their first turn purchases)?  If I believe that it's going to be a three-stack game, or I will be pushing it that way, there is no mid-game for me.  I'm usually going to try and buy as many VP cards as I can while keeping my deck's head above water.  Since I've evaluated the tableau and decided that there won't be an extended run on Provinces/Colonies, the bar for success in my deck has changed.  Trying to keep a consistent 5 purchase power (for Duchies) is a much different thing than trying to build to a consistent 8 or more, and in such a deck, mid-power cards can always be useful.

If I've decided that it's going to be a game where one of the big VP stacks is going to run out, then presumably I've decided how I'm going to drive me economy.  At every step of the way, what matters more to me than anything else is answering two rolling questions:  what is the average value of the cards in my deck and how does my purchase change that number.  Adding silver to a starting deck changes things from .7 $/c to .82 $/c, pretty good.  A Laboratory, added to a starting deck, does a little worse, .76 $/c.  But, later on in the game, if you've got a deck that is averaging 1.5 $/c over, say, 25 cards, adding a silver doesn't do much, 1.52 $/c.  But a Lab does better, 1.62 $/c.  Etc.  Obviously, I'm not computing this stuff directly at the table, but you should have a rough idea of where you are at and should only buy cards that can move the needle.  Stuff like Silver (or similar cards that don't have +Cards attached to them) is limited, and should be avoided once the average value of your deck is getting close to their inherent value.