Trip planning is so much easier if everything can be reserved in advance. Especially when coming from afar, knowing that everything is pre-arranged can ease your mind in very popular places like Grand Teton National Park. However, the campgrounds there are first come, first-served, making advance planning frustratingly impossible. The good news, though, is that there is a way to almost guarantee success in getting a site. Some of the campgrounds don’t always fill up, but the crown jewel of the Grand Teton National Park campgrounds, Jenny Lake, is ALWAYS full in the summer high season. Here is how the process works.
The key is to arrive early. Very early. We got there at 6:30 in the morning on July 10, and found a line of cars waiting at the campground entrance. We were fifth in line, and the line continued to grow after we arrived. Every day at 8AM, the campground host emerges, and talks to the people in line, one by one. You are told to drive around the campground, select a site with the green “open” tag on the site’s post, flip the tag to the red “occupied” side, and return immediately to the campground entrance to pay the $29 per night fee (cash or check). Most of the “open” sites still have sleeping campers occupying them at 8AM, and they do not have to check out until 11AM. So, after our arrival, we first went for a nice early-morning walk to the shores of Jenny Lake (don’t forget your bear spray even though it is a short distance) until the check-in, and then explored the visitor center before returning around mid-day to set up our camp.
The Jenny Lake campground has 50 sites. When you drive around, it is tempting to skip the first few sites to see if you can do better. There are plenty of nice sites in the campground, but we took site 1 despite the temptation to drive around more, because it was really a great site: beautiful views, one of the more private and quiet sites, close to the bathrooms, and easy access to the trails to the lake and visitor center. I can also tell you that the sites near the end are less good; especially bad is site 50.
I think most people at Jenny Lake stay 3-5 nights (we stayed 5). If we assume that the average site occupation is 4 nights, then each day 12 or 13 sites will become vacant. Of course, there will be some variability. But I think that if you are at position 20-25, your chances of a site are very slim. If you find yourself up to 10th in line or so, I would say your chances of a site are quite good. Good luck, and enjoy the beautiful place!
This is the sign that greets you. 
Waiting in line early in the morning.
Grand Teton National Park- Cascade Canyon hike