Know the terms of the contract
Once you've made your selection, make sure you read and understand the terms of the contract before signing anything. It should obligate the broker to implement the marketing plan and report periodically in writing, and it should allow you to terminate if the conditions are not met.
Some brokers may want you to commit to a six-month listing, but you shouldn't accept a term any longer than 90 days. You can always renew the contract.