For me, I'm most productive early in the morning and mid-afternoon when I get back from lunch. I'm worthless just before lunch and anytime at night.
That is, unless I get intoFlow. Flow is something that I definitely subscribe to. One of the responders to Todd's question also brought it up and, like me, feels it is random and unpredictable. I've found, though, that you can sort of force Flow by giving yourself deadlines.
Here is a good way to get into Flow every day... write down a task that you know can be handled in a short period of time, tell yourself you have to have it done by a certain time and start working on it. If you finish early then you gave yourself too much time. But if you get the timing right you'll be working really hard to get that task accomplished. Once you do, you'll be on a roll. You'll achieve Flow. Don't stop. Don't get up. Just start something else on your list.
I've found this has worked for me when I've needed to force myself into Flow.