OK, here are my words of wisdom. The Main Salmon is my favorite multi-day trip (closely followed by the Middle Fork). I have run it at least once a year for the past 10 years.
Guide books - the best one I've found is by Eric Newell, blackcanyonguides.com. Good info, GPS coordinates for camps, good descriptions and drawings of rapids.
Water level - The best way to find out the water level is to call the North Fork Ranger station. There is no gauge reading for Corn Creek on the USGS website. The highest I have run is 2.5 feet, so I have no real "high water" experience. My friends have advised that at high water the hardest thing is keeping your group together, especially if someone swims/flips & catching eddies for camps because of the river speed & smaller eddies. Some rapids wash out at high water, some get much harder. Elkhorn, Whiplash, and Chittam are a few that get nasty at high water. Anything above 4 feet is considered high water, and above 7 feet is considered extreme. There are two new rapids that formed last year.....Alder Creek & Black Creek. Black Creek is a kick in the pants, but it's formation has covered one of my favorite rapids.....Salmon Falls
Camps - The Main Salmon has lots of great camps. This river is on a partial reservation system. Certain camps can be reserved and the rest are first come first serve. Reserve camps are designated as small group (1-11 peeps) and large group (over 11 peeps). Sometimes if a large camp isn't reserved by a mid-day launch they will let a small group have it. You can't layover at reserved camps. Length of trip is also designated by group size..... 1-10 folks is 8 days/7 nights, 11-20 is 7 days/6 nights, and over 20 is 6 days/5 nights. Since we usually have a small group, we mostly use non-reserve camps and do 1-2 layovers. We are an organized (but not obsessive) group, so we usually are on the river by 9:30 am so that we can get our desired non-reserve camp and avoid the afternoon wind. One word on the non-reserve camps.....they are First Come First Serve. Don't write them down on your desired camp form at the ranger hut and think this means you get them. You only fill out that form for reserved camps. For the past two years we have had groups float up to our camp at 4-5pm and whine because we are already there. If you want to get a desired non-reserve camp, don't hang out in camp until noon. Ok....I'll get off my soapbox now
Shuttles - I have used River Shuttles out of Salmon for 10 years with no problems. Jet backs are common and can be fun if they work for you based on number in your group and your budget. Jet backs don't often run in late season at lower water levels.
Jet Boats - You will see them every day. If you treat them with respect they will do the same. If they see your group preparing to run a rapid, they will wait for you to finish. Be courteous and keep your group together and get through the rapid as quickly as you can so that they can go. It helps if your lead boat lets them know how many boats are in your group so they know when you are done running the rapid. If you are eddied out to scout, wave them on through. It's cool to watch them run the rapids. If you encounter a jet boat on the river (not in a rapid), have everyone pull towards the side of the river to give them room, but you don't have to eddy out. Jet boat folks can be super helpful if you have an injury/emergency on the river. In 10 years I have never had a bad jet boat experience. They are just part of the Main salmon experience.
Most folks run from Corn Creek to either Vinegar Creek or Carey Creek. We always take-out at Carey Creek so that we get to run Vinegar rapid. This year we are floating down to Spring Bar to add an extra day on the river. There is a scat machine at the Chevon station on Riggins.
Must see stops - Legend Creek pictographs, Barth hot springs, Buckskin Bills (ice, water, soda pop, beer, ice cream, t-shirts, and a cool place to see), Mackay Bar (more ice cream, drinks, t-shirts, flush toilet), Jim Moore, Campbell Ferry, Reho Wolfe, Polly Bemis, and Shepp Ranch homesteads.
Dogs are allowed on the Main Salmon. You will see a fair number of them and my personal dog experiences have always been good. We bring ours and she loves it. Clean up after them. Don't let them be annoying at the put-in & take-out. Don't let them chase wildlife. Be a responsible owner and they will have continued fun on the river.
Weather - It can be hotter than hell from July to September. But I have had cool wet trips in August. Be prepared. Bring shade tarps & lots of sunscreen. The water is awesome for swimming at camp.
Have a great trip and be safe