Text from WoWWiki:
The rogue poisons quests for Horde and Alliance provide the next opportunity for solid skill gains. The quest can be picked up at level 20, although it may be too difficult for many players at level 20. The item required to complete this quest is contained inside a locked chest that can be picked repeatedly up to 170. (You are likely to hit your skill level cap first, if you are on the poison quest.) If you've already finished the quest, you can still use the chests to gain skill. The chests don't highlight when you mouse over them after the first time (when you obtain the quest item), so you have to activate your lockpicking skill first and then click the chest. The chest will be empty if you already have the quest item or have completed the quest. Be sure to close the empty loot window in order to speed along the chest respawning (it is very quick if the loot window is not left open.)
Video accompaniment:
[ame="http://www.youtube.com/watch?v=l4UihOS5djs"]YouTube - WoW Easy lockpicking 90-170[/ame]