I usually use insoles..you can use every kind of them: anti-odor, wood, cotton...
The problem for me is that in summer shoes with leather insole (almost all of my flats and sandals) become very slippery...with these I don't have problems anymore. I buy them in drugstore, here in Italy they cost about 2-3 euros but i believe the price is almost the same everywhere.
For sandals i cut them in a way that they can't be seen...they would look very ugly otherwise