I tend to use an od green army issue triangular cotton "bandage". Common name, "Drive on rag", the bane of BtB soldiers everywhere, but like a regular bandana, tremendously useful. Somewhat bigger tho, and triangular.

When the powers that be finally regulated we were not to wear then around our necks in the field, many of use took to wearing on through our belt loops on top of out belts, just to have it handy.

Uses included:
Sling
Bandage
Weapons cleaning
Personal cleaning
Emergency TP (usually leading to the rag being retired)
Torn into strips for camo
Rucksack strap padding
Do rag under helmet in the winter to hold in a little more heat

and all the uses previously mentioned