Start off with two profiles and go from there. I use virtual desktops (spaces on OS X) to manage it. Space 2 is gmail, space 3 is logged in sites, space 4 is development, etc.

Once you get used to it you instinctively switch without thinking about it

I also created an app for OS X that creates temporary throwaway browser sessions for any supported browser you have installed:

https://github.com/nikcub/tmpbrowser

Makes it easier than using command line options and creating users