I have a couple Dad's Hat Rye Whiskey Barrels and a Woodinville Bourbon Barrel. If you aren't going to fill it right away, it's a good idea to keep it wet in a sanitary way. Put some whiskey in and change it's position each day, rotating from the head to the staves. If you don't want to move it everyday, fill with a holding solution of Potassium Metabisulphite (k-meta) and Citric Acid. 1 teaspoon of Citric Acid (5.25 grams) and 1.5 teaspoons of k-meta (11.25 grams) per gallon. Top up with water every so often as it evaporates. I do not like spirit heavy barrel aged beer so I opt for the Citric Acid and K-meta solution since it strips out some spirit flavor. Smaller barrels will contribute more flavor faster, they will also allow more oxygen in. I coat 2/3s of my 8 & 10 gallon barrels with paraffin wax to try and mimic the same surface area to volume ratio with respect to oxygen permeability as a 53 gallon barrel. When you are ready to fill, if you used whiskey to keep the barrel wet, just dump it out (but keep it!) and fill the barrel. If you used the holding solution, rinse thoroughly before filling. Age for a minimum of 3 months, 6 months should be better.