The Bite-Sized series of articles is meant to take the most powerful cards in Dominion and give a short explanation of what makes them powerful.
Ambassador is one of the most powerful cards in Dominion. The two most powerful effects in the early part of most games are thinning and junking – both contributing to deck control in very significant ways. Ambassador does both at the same time.
You usually want to open with Ambassador for this reason, either with another useful card that doesn’t get in its way, or with a second Ambassador. Your goal through most of the game is to just play Ambassador lots of times. You usually prefer returning two junky cards to one almost all the time, regardless if the one junky card is weaker (i.e., thinning two coppers is usually better than one estate or even Curse). Just make sure you don’t get rid of all the economy in your deck.
If your opponent is also playing ambassador against you, you need to be aware that there is often a snowballing effect – the thinner your deck is, the more often you can play ambassador to stay thin and junk your opponent. The thicker your deck gets, the harder it is to play enough ambassadors to stay thin. However, because you can return two cards, but only junk one, it’s possible to get to a point where both decks are stable and not growing too much. At that point, you can build a deck more or less normally, so long as you keep your deck’s size under control.
For more details, see this article or this podcast episode.