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.