Exercise frequently, especially going up and down stairs. The elevation is what will really get you, and there is no substitute for being in good condition. If I'm drawn on May 9 for New Mexico I'll be adding stairs to my regimen later this summer.
Buy your base, rings and scope now if you don't have them already. Get the rifle set up and start shooting. Practice out to 300 or 400 yards if you can. This will seem like a looong way if you've not done it before. A good rest will help immensely. If you use factory ammo you'll probably have to buy a box of 3 different types and see how they each group at 200 yards, then go with the best one. With some of the better factory loads you can sight in at 250 yards and be within 3 inches above or below your line of sight out to 300. That means that anything between you and 300 is just aim and shoot.
If you own a bipod, get used to using one. Also look into other types of portable rests such as monopods or similar that you can easily carry.
Buy the best binoculars you can afford. I'd rather take a $200 scope and expensive binoculars than a $1000 scope and cheap binos. Yes, there is a definite difference between $400 Leupolds, $1200 Zeiss and $2000 Swarovski binoculars (though the $1200 Zeiss is very nice).
Find a nice daypack style backpack that you can use to carry your stuff such as inflatable seat cushion, spare ammo, binocs, flashlight, knife, etc.
Wear no cotton.
Send me your e-mail address and I'll send you my packing list if that would help.