Foot powder is handy for "drawing" things on the ground. I've also seen a fellow carry little plastic bottles of cheap watercolour paint powder for the same purpose. The disadvantage is that these things get used up, while wool, string, little army guys, etc. can be reused.
Best way to approach this is to think about what it is you believe you're going to have to model, then use your imagination. All of the suggestions so far are good; I carried wool, nails, little soldiers, laminated 3x5 index cards as mentioned, as well as popsicle sticks (good for "sculpting" sand and dirt, and can be used to represent things), foot powder (which I was carrying anyway), grease pencils (to write on the cards; wouldn't smear with sweat/rain), and some coloured wooden blocks (blue, red, white, green...they were just off-cuts of 1x2 that I painted with cheap enamel) to represent various things (vehicles, bunkers, buildings, etc.)
Don't forget to discuss your map modelling with your 2ic, and maybe practice making a quick model with him/her, so it gets built the way you want. Your 2ic will, after all, usually be the one building your model for you, while you carry on with battle procedure.