Manage board space, hand space, and your deck
Leave room for the cards and minions you want. A full hand or board can waste part of a play.
Board space
Each player has seven board slots. Minions and Locations share them. Even a small minion can block a larger summon.
Count the spaces a play will need. A Deathrattle may fill a space after a minion dies. Think about the final board, not just the first step.
Hand space
Your hand can normally hold ten cards. If you draw with a full hand, the drawn card is destroyed. Cards made by other effects may also be lost when there is no room.
Suppose you have nine cards. You play a spell that draws three cards. Playing the spell leaves eight cards in your hand. The first two draws fill your hand. The third is lost, unless another effect changes the result.
Playing another useful card first could make room for all three draws. Count cards made by extra effects too.
Cards left in the deck
Drawing helps you find what you need sooner. It also brings you closer to an empty deck. Trying to draw from an empty deck causes Fatigue damage.
In a long game, check both deck sizes. Ask which threats and answers are left. More cards help only if you have the time, Mana, and space to use them.