Spend Mana with a plan: curve and development
Use Mana to build a useful board and keep your plan moving. A good Mana curve helps you make plays on early and late turns.
What is a Mana curve?
Your Mana curve is the spread of card costs in your deck. A fast deck needs early plays. A slower deck still needs ways to handle early threats.
A cheap card is not always a good first-turn play. A removal spell may need an enemy target before it helps.
Plan the next turns
Suppose your hand has a two-cost minion, a three-cost minion, and a spell for a certain problem. Playing the two-cost minion on turn two can set up the three-cost minion on turn three.
Using the spell just to spend Mana may waste an answer you need later. Good plays fit your plan as well as your Mana.
When to save a card
Unused Mana does not carry into your next turn. Even so, leaving one Mana unused can be better than wasting a key card.
You may want to save a cheap combo piece or a spell for a bigger threat. Look for a safe turn to play a threat of your own. Later, you may be forced to spend all your Mana on answers.
Check your deck over many games
If you often have nothing useful to play, your curve may need work. One bad opening hand is not enough reason to rebuild the whole deck.