Balance your terminal actions (ones with no +actions) with your villages (cards with +2 actions, and possibly more). Oftentimes, I see new players buying a bunch of terminals that they wind up drawing together and can't play, or on the other end, buy a bunch of villages when they have no use for the extra actions. Villages and terminals should be around 1:1 in your deck, with a couple extra villages perhaps if they're terminals that draw cards (drawing dead is a waste of both a card in your hand, as well as that action play for the shuffle you're in).
I can't agree with this advice. Except in the vaguest sense of "balance your deck composition". It is almost always better to have a deck with too many terminals, than too few. So one should aim for a couple extra terminals, not a couple extra villages. In a treasure+terminals kind of deck, you should not be afraid of some terminal collision, you should be afraid of having a turn where you don't play your terminal.
I think the worst trap that new players tend to fall into is too many nonterminal actions. Which are generally much weaker than terminals, but feel safer and allow the "chaining" of actions that can feel quite nice, but is often just spinning wheels. But this mistake happens to all players at all levels, and most people could improve their overall game by buying more terminals.
Expanding on this a bit:
When villages are on the board, you need to ask "How important is it to play more than 1 terminal a turn?".
Sometimes your strategy will require that you play lots of terminals each turn, for example when you want to draw your entire deck with Smithies. Given that these sorts of decks take time to build, you want to be sure that you have a large enough source of +actions to draw the cards you need to be able to have awesome turns (perhaps one where you buy multiple Provinces to catch up against a player buying mostly money and VP cards).
But sometimes, you might play game where the maybe the only terminals are Chancellor, Militia, and Moneylender. None of those cards really need to be played more than once a turn. It is good, however, to be able to play one of those cards every turn (specifically, the Militia). If you can't draw most of your cards with Laboratory or something, then you should consider "Over terminal"ing a bit to ensure that a powerful card is played almost every turn (do expect collisions though). If you need to space those cards out a bit, then just buy some Silver or Gold.
If you can draw pretty much your whole deck with cards that give +action +cards like Laboratory, then you don't need more than a couple of copies of the terminal you want to play every turn. The extra copy is for when your deck starts to get clogged with VP cards.
