Keep your powder dry
Definition 1 of 2
Pro Tip 1/3Resourcefulness
It implies saving something valuable for a more impactful moment or when it's genuinely needed. 
She would argue now, but she chose to keep her powder dry for the bigger debate.
Pro Tip 2/3Strategic Patience
It suggests being strategic about the timing of your actions for maximum effect. 
They kept their powder dry before launching the product at the optimal time.
Pro Tip 3/3Not Just Literal
The phrase isn't about actual powder; it's a metaphor for any kind of preparedness. 
In negotiations, he keeps his powder dry, holding back some arguments.