On my PC I have installed, Mandriva, Ubuntu, Gentoo, Sabayon and Debian (my main Linux operating system), but when I turn it on, usually I did not get the grub menu, and I am forwarded directly to Debian, sometimes I have to restart the system to start another Linux flavor.

So, the solution was to put the timeout of grub menu to 15 seconds, so I have enough time to react even when I am not paying attention to the PC startup.

The timeout parameter is stored in the file

/boot/grub/menu.lst

So, to change it edit that file with your favorite text editor. (mine is vim)

sudo vim /boot/grub/menu.lst

and look for the line that contains the string "timeout"

This is how my file looks

## timeout sec
# Set a timeout, in SEC seconds, before automatically booting the default entry
# (normally the first entry defined).
timeout         15

please note that the above text is only part of my menu.lst.

If you want grub to wait for you to take an action before continue, comment the timeout line, or erase it.