Roy wrote about tips for a successful team lead. These are things you can start implementing tomorrow morning.

I think the Demo is great to create a constraint for the team. James Shore talked about that in his pre-published agile book.

I would also add time-boxing with short iterations. When the time to release is 6 months, the first 2 months everyone still relax from the last release overtime, and the last 2 months are full of integration pains an more overtime. Rinse, repeat.
Short iterations require you to focus and not wait (known as the Student Syndrome).

One thing I wouldn’t do as the FIRST thing (today) is put the overtime limit. Although this will work, it needs the political climate to gain approval from higher ranks. You can do this after you have gained credibility and showed results through other methods. After that you’ll be able to do whatever you want.