So many people have good advice and they all touch on the fact that the power of a card is relative to what other cards are also on the table. Due to this, it is hard to give universal advice that is more specific than that. However, I will try.
I was actually trying to improve the game of my wife tonight, and here is what I told her. There are three types of action cards:
1. Terminal Actions. These are actions that do not have at least a +1 Action on them.
2. Non-terminal Actions. These are actions that have +1 Action on them. For the sake of this post, they have exactly +1 Action on them.
3. Facilitator Actions. These are actions that have +2 (or more) Actions on them. These cards allow you to play multiple actions afterwords, which allows you to chain Terminal Actions.
Looking at the action cards like this, it is easy to divide your strategy into three different types.
1. Pure Terminal Actions. If you are buying just treasures and Terminal actions, you must limit the amount of action cards you buy. Your goal is to never have more than one terminal action in your hand at once since any addition ones would be dead cards.
2. Pure Non-terminal Actions. You can have as many as you want and you will ALWAYS be able to play them. ALWAYS. That is important to note, because even if they don't do a lot for you, you will always be able to at least play them. You should still be careful about getting too many non-terminal actions that do not have +1 Card as unless they also give +x coin (or some other good factor) your buying power will suffer.
3. Combining Terminal Actions with "Facilitator" actions. People see Smithy and Village and go "oh yeah!" Then, by turn eight, they are wondering why they are not doing as well as they do when they go pure Smithy. Combining these two types of action cards is much harder than most people think. In this case, you MUST play your Village first, or it becomes a dead card. Say you draw a Smithy and four coppers, you play your Smithy and draw another Smithy, a Village, and an Estate. All three of those cards are dead. Sure, your deck has the POTENTIAL to go Village -> Smithy + Smithy, but what are the odds of you actually getting your Village first and having at least one Smithy to play after that? People see a combo that could work amazingly well IF they trust in the heart of the cards, problem is, this isn't Yo-gi-oh, and you're no Pharaoh.
I've have, and have seen some amazing combos being pulled off by combining terminal and non-terminal action cards, but I have also seen people (including myself) attempt to do so and fail miserably. So, I end with a KISS. In English class, Math class, and in life, keeping things simple is usually the best thing you can possibly do. If you ever find yourself trying to decide what strategy to use, pick the one that requires the fewest action cards and you'll usually play a better game.