Isn't throwing considered a ranged skill, not melee? So basically take an archer build, and substitute throwing.
The direct answer to the poster's question would be "Tactics", to increase the damage done, just like it does for all other weapon skills. It also opens up the first special move when you have 70 Throwing and Tactics, and the second special move when you have 90 Throwing and Tactics.
If you read the Stygian Abyss documentation quickly, you may wonder if you need Swordsmanship and Parry to go along with Throwing. The answer is no, not if you want to be a ranged fighter. Swordsmanship will reduce the accuracy penalty Throwing has when foes are too close. Parry will reduce the parry penalty when fighting at melee range. But if you plan on being a ranged fighter, you don't need either of these. If you want to be a melee fighter, skip Throwing altogether, IMO.