You can download this:
http://www5e.biglobe.ne.jp/~arcana/software.en.html
It installs a preference pane in your system preferences that allows you to set the volume of the startup sound, including mute it.
I installed it because in the beginning, I also always shutted down my mac after using it, but for some strange reason, it never used the volume setting I had for the startup sound, it was always at full blast (pretty annoying late at night).
But 2 things I'd like to add:
1. Don't mute it completely, because it's also an indicator that everything's fine while starting up.
2. Use the Mac's sleep function. Unlike most Windows PCs, it works very good, the Mac starts nearly instantly, plus the pulsing powerlight looks cool IMO (well, not if your Mac is in your bedroom though...).
I always put mine to sleep, as most Mac users do. The only restarts I do is when an Apple software update requires it.