To change the speed at which Googlebot crawls your website, you should first register at:

http://www.google.com/webmasters/tools

Once you create your account, log in to it and go to Site configuration > Settings > Set custom crawl rate. There you will see a slider through which you can set the crawl rate.

To reduce Googlebot's crawl speed, move the slider left (Slower). To increase the crawl speed, move it right (Faster).