Who Can Sign - For partnerships, only general partners can sign a contract, not a limited partner. For LLC's, a managing member or a hired manager can sign. For corporations, the company's president or chief executive officer is presumed to have the authority to sign. For an organization or association, a board president would have the authority, but may require a vote of the governing board to approve the deal.
Naming the Parties - You begin the contract by naming the parties in the agreement. This may seem obvious, but attorney Mark Cohen of Colorado writes in his article Ten Things to Consider Before Signing a Contract that "a common error is to use the name of a person representing an entity rather than the name of the entity."