Practice.. Grab your gear and hump it around your neighborhood. I used to get weird looks carrying a decoy bag full of gear and a sledgehammer (simulate a gun) around the neighborhood, but it helped. Get you body used to carrying the weight. Nothing sucks more than not being used to it and halfway through your walk-in and your hating life and then realize you have to go back after you're done. Every hunt you will learn. You'll tweak how you hump your gear, and what you bring every time.
Nothing worse than a "pack in" that's out of shape.
Another tip, get some friends that pack in, usually gear can be split up and lighten everyones load. When I take friends to a public walk in, I carry everything (I think it gives them an idea on how hard the walk is when your only carrying a shotgun)