Look for card economy as you build decks. Advanced players sometimes use the term "Min-Maxing" to describe the process of finding the most valuable cards for their strategy and matching them with the best ‘helper’ cards. They minimize the number of cards necessary to make their strategy work, while maximizing the offensive and defensive power of the deck.
Good advice.
This is especially important in how R&R lets you build decks. There are no minimums or maximums in how many cards you can bring into battle.
Bring too few and you'll run out, taking a 50% health hit. You'll get your cards back, but it's next to impossible to win after this penalty. Take too many and you won't be maximizing your odds in a random draw.
Try having lots of decks of various sizes. Playing some who is a lot stronger than you? Chances are the game won't last very long so it makes sense to only bring your best cards (but beware of running out of mana!). Playing someone who is at your skill level? Then the game will probably go the extra rounds and you'll need a bigger deck to avoid running out of cards.
Lots of strategy involved in figuring out exactly how many cards to bring with you to beat specific opponents.