How to force Startup Manager menu to appear at every boot?
I'm not sure this is possible by default without holding down the ⌥ key at startup. You can make the Boot Camp partition always boot by selecting it in System Preferences → Startup Disk, but it sounds like this isn't what you want.  

However, if you're not adverse to installing additional software, I think rEFIt may do what you need. The section on Getting into the rEFIt menu says:


  If you have installed rEFIt on your Mac OS X volume, it will be loaded
  automatically when the Mac starts up.


You should then be able to select whichever OS you want to boot from there.