Usually when players are losing to Big Money, they are either buying too many action cards, or buying action cards that don't really contribute to an overall strategy.
Next time you play, pay attention to two things. First: do you often have hands in which you have at least one action card that you couldn't play that turn because you didn't have extra actions? Second: after you finished playing your actions, were you able to buy something better than you would have if you had just skipped your action phase?
The first question will tell you if you are going for too many actions. There are many exceptions to this rule, but if you often find that you don't have enough actions to play your action cards, then you generally need either additional +action cards in your deck or fewer actions overall.
The second question is asking if those actions are working for you. This is much tougher to assess: there are some actions, like Militia, that don't improve your hand more than a silver would in their place, but they do hurt your opponent. There are lots of other situations where an action card will improve your hand, but having a different card for that same cost would have been even more useful. But by looking at how much better your hand becomes when you play certain actions, you can hopefully start to see which actions are contributing towards your strategy and which aren't giving you the most value.
