Heh, that sounds like the first sling I made, with a square of stockinette fabric and some string.  I had to keep re-tying the knots after each throw.

For quick slings, I'd recommend the sling knot with a piece of cordage at least six feet in length.  I bought a 50' length of braided nylon rope for three bucks at Wal-Mart and using only some scissors, the sling knot, and bowlines, made three servicable slings in about five minutes for use at a lake.  You can read about the sling knot here, on the forum:

http://www.slinging.org/forum2/yabb/YaBB.cgi?board=1;action=display;num=11110954...

This sort of sling has always worked handsomely for me, even despite its quick nature.  And for clarity, the "sling knot" I refer to is the locked sheepshank mentioned by Willeke later in that thread.  I haven't tried the one Yurek uses, but it looks similar.

Ah!  Thank you for clarifying that, Willeke.  Truly, though, the locked sheepshank has served me well with golfball-sized rocks, or their equivalent in weight.