Blackjack is the most reliable knockout skill. It doesn't last as long as bard/mage sleep, but it doesn't care about saves. Once perfected, you will rarely (if ever) fail steal on a sleeping person.

Thief waylay is also useful. Especially for a pickpocket, who is much less likely to fail to steal from a non-sleeping person. If someone's inventory is so huge you don't think you'll be able to clean them out during a sleep duration, then waylay first, steal for a couple hours, blackjack, then steal the rest. They can't call for help (IC) while waylaid.

Pickpocket also gives you bash/throw protection.

You will find it very difficult to kill people, though, and if you steal regularly then (some) players will decide you're a griefer. Expect to get full-looted whenever you die. If you're successful, expect certain players to assemble OOC gank-squads specifically designed to make your life hell.