Website, DB, & Software Developers

In writing:

The definition of an acceptable initial site, the workflow defined, every function they want it to do listed, you are complete if you check off everything. If it's not on the list, it doesn't get done.

Written design approval at an early stage, single point of approval for design (you do not want to be dealing with multiple people with different, conflicting ideas)

The definition of an initial upload of inventory, the date by which the owner is to have the inventory available, what fields will be in the Excel spreadsheet, how many you will upload.

Maximum hours of staff training, maximum number of sessions, last date training will be available under the initial contract.

The rate at which subsequent training and/or services will be charged.

I've worked on teams doing contracts for millions of dollars. You can seriously get screwed if you don't have all of this down. A client can decide to not accept what you agreed on, or just keep changing his mind, causing you to re-do work constantly. You won't believe how many of them don't really know what they want, so it's up to you to get them nailed down.