wedgenbiggs

Depending on base I know some who have roughed the surface with a course sandpaper

What Glen said.  Rough them up with some 120 grit.  Same as cowboy boots.  Assuming they're leather soled.  Or walk around on your driveway for a bit.

You can buy grip pads to stick on the soles they are cheap and easy to find

I used the iron on interfacing on mine. Works great, easy to apply and inexpensive.
 

Thank you! I couldn't think of the material's name for the life of me.

That is also quite a good idea, like this you can use different tupperware containers for different costumes and just carry whatever you need for a troop/convention. I will probably carry all my stuff in this one case. (until I need a second one )