Every client engagement should be properly documented with a contract. That’s the surest way to ensuring you can handle any surprises that may crop up in the relationship over time.
A standard approach is to have a Master Services Agreement (MSA) that includes all of the terms of the legal agreement, with individual Statements of Work (SOW) that define specific projects or other business terms.
This approach simplifies the addition of new projects down the line and hopefully takes the client’s legal department out of the mix on many new business opportunities. They will certainly want a heavy hand in the MSA, but most take a less in-depth review of new SOW’s on the same master contract.