With short range Skirmish weapons (e.g. javelins, darts etc) it's best to put them on Hold Formation rather than Skirmish otherwise just as they get close enough to throw them they back off to safety However be careful they dont get caught in melee. I normally only use them on an enemy armoured unit that is already engaged in melee. For best results fire javelins into the enemies flank or rear and watch the body count go balistic Jinettes are really good as they pack a powerful punch when charging so when they've finished lobbing javelins in the enemies back charge in for the kill or hunt them down when they rout

Skirmish mode does work well with horse archers or mounted crossbowmen though, i like luring enemy spearmen miles away from the main battle continuously peppering them with arrows and exhausting them

If you have a decent line of spearmen then putting Kerns or Spanish Javelinmen in a thin line behind the spearmen (on Hold Formation again) is another good way to slaughter armoured troops.

And as said by Rocket-Boy, use to force them into melee.