Get everyone who counts to agree up front and sign an agreement that says,
"This is what it will look like and this is what it will do. If it does this, sounds like this, looks like this, you will sign off on the project. You will not change the 'Quit' button to 'Exit', you will not change the background color to mauve. It will ship as designed".
I know, this sounds so old school and it is. But agile or "just in time" or whatever you want to call it only works only if the project is well defined. Having the designer sitting beside the programmer without a defined project simply results in more thrashing faster. Can you imagine building a skyscraper and designing it as you go? Been there, done that, don't want to do it again.
So how do we get our arms around this? First define what is a project. A project is not 'create Facebook'. For the purpose of this discussion let's call that an initiative. A project is 'add pictures to the Facebook Friends list'. A project is not 'convert all the offices to wireless'. A project is 'convert the network at the Fifth Street office to wireless'. A project is a defined piece of work that can be measured with regard to the requested design.
Seth Godin has a great podcast that explains this far better than I can in a one page blog. Listen to Episode 14 - The ShipIt Journal. Here Seth lays out from experience why getting sign-off up front is so important. Here's a link to referenced The ShipIt Journal as a PDF.
Go, ship it, on time.
