After setting the fee rate with
settxfee -- how do I re-allow bitcoind to set the fee automatically again? Or does the
settxfee command only apply to the next outgoing transaction?
Looking at the source code suggests that
settxfee 0 should return you to the automatic fee setting. However this behavior is undocumented and there is a TODO suggesting that it might go away in the future. Otherwise, you can restart bitcoind.