I'll second the gaiter recommendation. I have been using gaiters of one type or another for over 50 years. I never have trouble with stuff getting into my shoes (or boots).

If you wear boots, you can make simple trash-stopper gaiters from a pair of worn out socks. Just cut the foot part off of the sock and run a zig-zag stitch around the cut edge. You now have gaiters.

To use, just slip the elastic tube over your foot and then put on your boot. Roll the elastic down to cover the boot/ankle junction and you are set to walk.

There are many types of gaiters available for purchase; check REI or your local store. If you prefer running shoes to boots, get a pair of Dirty Girl gaiters from http://www.dirtygirlgaiters.com/ and use them. I wore DG gaiters on a recent through hike of the JMT and never had stuff get into my trail runners.