For lawyers' scope, creep is easily dealt with. 

The law requires that a fee estimate for work is given to a client. To provide a fee estimate the scope of work must be set out in the retainer.

If work is requested outside the scope then an additional retainer for the new work is issued to the client to approve before the work is started. 

To avoid scope creep the same process can and should be adopted by all trades and consulting service businesses.

Bt having a signed contrract that sets out the price and any additional work will be charged at an hourly rate.

Be clear in your quoting, what is IN and what is EXCLUDED; listing both always helps. Ok sure, It isn't always possible to list everything. But always manage expectations. Then be flexible; Absorb where you can, but stay within commercial viability. Having the conversation with good clients, always ensures both sides are understanding. The earlier you manage expectations the better.