I like the idea of sending a generic letter home for the parents. Often times when I try to call home I reach an answering machine, so this would be a good way to reach a parent.
At the end of each semester during locker clean out we keep the better shorts, shirts, shoes, warm-ups and other apparel, and have them cleaned. We store them and if students forget or simply do not want to dress or perhaps they cannot afford to buy clothes or shoes, we bring them out and they participate.
It is to the point now with our students that they know we have clothing and shoes so they dress instead of trying too get out of certain activities.
If they refuse to utilize our clothing or shoes then a call is made home to the parent/guardian and this usually takes care of the problem.