I have a little no-name insulated cooler bag I picked up at our local grocery-department store. (I should have gotten the next size up - big salads don't fit!)
If you eat yogurt, I've found a great way to keep my food cold is to freeze my yogurt, then nestle my fruit up next to it.
It has the side benefit of changing the yogurt's texture so it's a lot creamier. Regular yogurt takes on a similar consistency to the custard-style - but it's still LF or FF!
A side benefit of the cooler bag is that my food doesn't pick up weird odors from the science experiments in the communal fridge.