How to put client content delivery schedule in contract

Hello, newbie here, first post. I look forward to reading through these forums.

I've been running into a problem lately where clients are dragging and delaying in delivering their content to me on website projects. I have kind of a system where my costs are vaguely stipulated on how much time I plan on each project taking, and lately it's been like drip drip drip with a page here and a page there, then a new version of some page, and on and on with editing pages and delays.

How do I put something in the contract, or statement of work, that details that they need to send me finished content in a timely manner, or pay a penalty or something?

I also have a business model where my project cost is fixed for a certain type and size of website. So they pay up front, or 50%/50%. so I don't know how to charge them a penalty later...

Any tips would be much appreciated!