Workflows

I have a question readers. How do you like to implement workflows? I am working on a project that has a workflow that I know is going to change and I am trying to figure out how I can best model this in code but still keep it supple.

My workflow is something like this:

Capture message
Decide who gets an alert
Send alert
--time passes--
User logs in and performs work based on alert
User marks work complete
System send alert to message originator
Bill message originator
++workflow completed++

So first off, one of the questions I am trying to answer is how to avoid building something I am not going to need. But beyond that:

How do you implement workflows?