An easy way to set the default is to edit /boot/grub/menu.lst:

Code:
gksudo gedit /boot/grub/menu.lst
Look for the line towards the top that says "default 0" and change it to "default saved". Then find the line towards the bottom of the huge comment block that says: "# savedefault=false" and change "false" to "true". Save the file and then run:

Code:
sudo update-grub
The next time you restart, whatever kernel you choose will be become the default for future restarts.