If I had to guess, I would guess maybe a quick Apache style overhand throw for fast accuracy while hiding his sling till the last moment from about 25 to 35 yards.  This would give him time to reload and throw again if needed before Goliath could realize what David was doing and close to javelin range.  Best to take Goliath down from beyond Goliaths effective range but not so far that accuracy would drop off to much.  (Although I do like the one about Goliath falling down laughing!)