Keep your powder dry
Be prepared and save your resources until they are needed.
The allusion is to gunpowder which soldiers had to keep dry in order to be ready to fight when required. This advice reputedly originated with Oliver Cromwell during his campaign in Ireland. In Ballads of Ireland, 1856, Edward Hayes wrote:
"There is a well-authenticated anecdote of Cromwell. On a certain occasion, when his troops were about crossing a river to attack the enemy, he concluded an address, couched in the usual fanatic terms in use among them, with these words - 'put your trust in God; but mind to keep your powder dry'."
19th century citations of the phrase invariably give the full version - trust in God and keep your powder dry. This emphasizes that the keep your powder dry was seen only as an additional insurance. This is made clear in a piece from The Times Literary Supplement, 1908:
"In thus keeping his powder dry the bishop acted most wisely, though he himself ascribes the happy result entirely to observance of the other half of Cromwell's maxim."
See also: the List of Proverbs.