Reseting computer cache/ Shift doesn't work in safe mode

Hi,

I need to start my MacBook Pro in safe mode. I know you are supposed to press long Shift when the sound goes off at the start up. The problem is that my shift is disabled as a hot key. Is there any other way to do this, or how can I enable the Shift key?
Or is there any other way to reset the computer’s cache


P.S. I need that so I could complete changing my Finder icon.
I got as far as changing the new icon to .png in the Resources, but in order for me to change it on the dock,
I have to reset the computer’s cache, which I could do by starting it in safe mode. Is there any other way to reset computer cache?
Any suggestions??

Thanks,

Alexandra