An often overlooked feature in 2.4.23+ and 2.6.6+ Linux kernels is the laptop-mode. It may be activated by writing a "5" into /proc/sys/vm/laptop_mode. Laptop-mode, when configured correctly, can make the kernel buffer disk activities for quite a long time and keep the harddisk spun down for most of the time to save power.
There is also a set of userland tools made to automatically manage all aspects of laptop-mode configuration according to the actual mode of operation (ac/battery-status). It is called laptop-mode-tools and you can install it in debian via apt-get or download it from here.
Almost anything you need to know about laptop-mode can be read in your Linux kernel documentation at /usr/src/linux/Documentation/laptop-mode.txt
If you can not find this file on your laptop, then go and get yourself the latest kernel sources.
Note that on some laptops the "hddtemp" daemon will wake up the disk every minute, and must thus be disabled for power management to be effective. Unlike other processes, hddtemp's accesses are not visibile through the /proc/sys/vm/block_dump facility. (This issue was observed on a ThinkPad T43 2686-DGU with Fedora Core 4.)