How can I boot in "Safe Mode" with rEFIt I would like to boot safe mode but I can't because of my rEFIt. 

Holding the Shift button at startup has no effect, I just get the rEFIt menu and OSX starts to boot normally.

any ideas how to do it?
The question on this page appears to be the same issue. Someone responded with a detailed solution, which might work for you as well.