As you play more and more Dominion games you start to develop a better feel for the cards that are in your deck. Maybe you can even remember every single card that's left in your deck at every point during the game. If you do, good job, Rainman! If you don't, like me, you can at least have a general idea about what's left.
The reason I'm writing this is because it's important to think about your game in turns of shuffles instead of turns. If you open Silver/Silver, it doesn't matter if you get a Witch on turn 3 or 4, because in both cases you can only play it on turn 5 the earliest, which is after your reshuffle.
I find myself often having a short term vision and evaluating my deck on a turn by turn basis. First turn after a shuffle: I have $6 now, hmmm, my deck really needs a <random decent $5>, so let's get it now, I might not be able to later. Then the second turn I'm looking at $5 and figure that my deck can't really handle or needs another <random decent $5> and have to settle for a $4 or Silver. If I had turned the order around, I could have gotten a Gold and that $5 I wanted.
So this is what I mean by thinking in terms of shuffles. Rather than thinking: "Which single card do I need now?" it might be better to think "Which cards am I looking to add to my deck this shuffle?" If I don't buy X now, will I be likely to get it later this shuffle? Of course this adds a bit of uncertainty, since you could get unlucky and not be able to get X at all. I think it's at least worth thinking about it and sometimes taking that chance will pay off.
Now I can't really give you any specific advise in the form of: When you have this much money and this much is left in your deck, get X, otherwise, get Y. I'm merely jotting this down to get you to think about the timing of your purchases.
Do you really need to pay much more money now, because you really want that card, or is it better to wait as you can afford it later this shuffle anyway? For example you might have a lucky $8 early and think "well, I want at least 1 Gold before I get a Province", grab a Gold, and have $6 next hand. Reversing this order you could have gotten a Gold and a Province, which in retrospect fits your requirement of "at least 1 Gold before Province", well, the "before" is just awkwardly worded now.
It takes a lot of experience to develop a good feel of what's going on in your deck economy wise and whether skipping that "crucial" card is a good idea, but I'm sure it pays off in the long run. You just feel so silly when you needlessly overspend for something you could have gotten a turn later as well.
