2–6 Months Before

Pick your park and check its alert pageWildfire smoke, road washouts, and seasonal closures (like Going-to-the-Sun Road in Glacier) can shut down whole sections for months.
Book lodging and campgrounds as soon as your dates firm upIn-park lodges and popular campgrounds (Yosemite Valley, Grand Canyon rim) often book 6–12 months out and don't take walk-ins in peak season.
Check if the park uses timed-entry reservationsYosemite, Zion, Arches, Glacier, and Rocky Mountain have run separate ticketed-entry systems on top of the entrance fee; missing this means you can be turned away at the gate.
Enter permit lotteries for specific trails or sites if specific trail permit neededThings like Half Dome cables or Angels Landing in Zion require a separate permit won by lottery, applied for months ahead.
Apply for a backcountry permit if you're camping outside a campground if camping outside campgroundMost parks require this regardless of how remote the site feels.
Buy an America the Beautiful annual pass if you're visiting 3+ parksIt covers entrance fees at all federal recreation sites for a flat yearly cost, cheaper than paying per-park after two or three visits.

2–4 Weeks Before

Map your route and fuel stopsGas stations inside and near many parks are sparse or nonexistent; fill up before you enter.
Download offline maps and the park's NPS app contentCell service is unreliable to absent in most parks, and paper maps at the visitor center are a solid backup.
Reserve a shuttle seat if the park's main road is shuttle-only if shuttle only roadZion Canyon and parts of Denali close to private vehicles in season.
Check the weather forecast for your actual datesMountain and desert parks can swing 40+ degrees between day and night, and afternoon thunderstorms are routine in summer.
Research altitude if the park sits above 7,000–8,000 feetPlan a slower pace and extra hydration the first day or two; altitude sickness can hit healthy people with no warning.
Pack layers, sun protection, and real hiking shoesFlip-flops and cotton are the two most common regret items on trail.
Check pet policies before bringing your dog if bringing a petMost parks restrict pets to parking areas, roads, and a few paved paths, not trails or backcountry.

Pack a paper map and a physical first-aid kitDon't rely solely on your phone.
Rent or borrow a bear canister if required if bear canister requiredMany bear-country backcountry zones mandate hard-sided food storage; rangers check.
Tell someone your itinerary and expected return timeEspecially for any solo hike or remote camp; this is what gets search-and-rescue moving if you're overdue.
Rent or carry a satellite communicator for remote areas if remote area travelCell service gaps in places like Big Bend or backcountry Denali can be total.
Load your entrance pass, camping confirmations, and permits onto your phone and print a paper copySome gate rangers and remote check-in stations don't have reliable data to verify online.
Pack out bags for trashMany parks have limited trash service, especially in backcountry and shuttle-only zones.

Stop at the visitor center or check the app for current trail and road conditionsConditions change daily with weather, wildlife activity, and repairs.
Keep food and scented items in a bear box or canister, never in your tentThis protects you and keeps the animal from being euthanized after it learns humans mean food.
Stay 100 yards from bears and wolves, 25 yards from other wildlifeAnimals that look calm are still wild, and habituated animals get put down.
Never feed wildlife, including "harmless" squirrels and birdsIt changes their behavior permanently and often ends badly for the animal.
Carry more water than you think you'll need and treat any water from streams or lakesMost park water sources are not safe to drink untreated.
Turn back if weather turns or you're not keeping pace with your planned scheduleExposed ridgelines and canyon slot trails are the classic places people get caught by afternoon storms or run out of daylight.
Stick to marked trailsCutting switchbacks erodes fragile terrain that can take decades to recover.
Check fire restrictions before lighting a campfire if lighting a campfireMany parks ban them entirely during dry seasons, with real fines for violations.