Closing lid when connected to monitor

I'v connected my macbook pro in my room with a monitor. I have a USB keyboard and a wirless Mighty Mouse. I know that you can press the power button when the lid is down on the keyboard and that works but then after about 2 seconds my desktop screen goes black. For example: I close the laptop screen when it's connected to the monitor and press the power button on my Belkin keyboard and the laptop screen pops up for about 5 seconds and then goes black again.

So my question is... How can I make the screen stay up on my monitor for more than 5 seconds?

Thanks!!!