Get a beer in it as quickly as you can. A strong beer. Until then keep the hogsheads (ends) wet. I got a newly used one from a distillery and kept it a month this way before putting a beer in it. I actually kept mine in a spare shower and would water it for 5 minutes once a week. Then I would turn it over on the other end the next week and repeat. A kiddie pool and a water hose could accomplish the same thing.
After the first beer was done aging in the barrel I did wash it out with hot water before putting the 2nd beer in. The whiskey character was greatly diminished after the second beer so for subsequent batches I have continued to wash the barrel out with hot water, but then I pour a 5th of whatever spirit I want to get flavor from (I have used bourbon, rye whiskey, and dark rum - considering port next) and let that soak in for a few days before adding a beer. So far it has help up pretty well. No drying out and no sour flavors.