Just be sure to load up the pack with your gear (including the equivalent of food, fuel and water) as soon as you get it and go for several hours' hike around the neighborhood (if it's good weather, and making sure keep the pack clean) or even around the house (what I did, since it was pouring rain the day my pack arrived). Yes, a very boring hike, but you need to be sure the new pack is comfortable for you and your gear while you can still return it!

I hope you enjoy your new pack!