6-8mo before your proposed conference date. You need to find a weekend date that is not directly conflicting with any popular conferences. In the summer, this can eliminate 80% of the available weekends. Once you have a date that seems alright, start looking for the campsite. The three criteria are: 1- it must be a group site (usually 30 is a good site size), 2- it should not be exhorbinant (~<$100/night is good), 3- it should be located somewhere that most schools can get to in fewer than 7hrs, and 4- it needs to be bookable. This tends to eliminate Yosemite, for instance, because it seems impossible to reserve the campgrounds there no matter how far in advance you plan. Book this campground early.
2 months in advance: Send emails out to people at schools who might be able to help you coordinate the outside schools. Easiest way to get participation is knowing someone who will act as a point person. Ask the previous Sierra Conference person for that list. Most Berkeley students know someone everywhere, so you can also ask them to get in contact if you don't know someone from a particular place. This is also a good time to email the whole department to whip up interest. It might also be a good idea to make a little website for people to look at, with a map of the campsite and some general information.
1 month in advance: You should be getting an idea of how many people are interested from Berkeley, and have your point people set up. This helps you get a head count to start thinking about the car situation. This is also a good time to ask the chair of the department how much funding you can get. It gets the event on the chair's radar to help avoid problems after the fact.
2 weeks: Okay, you now want a more definite headcount. Sometimes there are not enough cars for everyone who is going, and so you need to think about renting a car. You should know how many other schools have people coming, and if there is one school with a lot of people, it is reasonable to ask them to make one of the meals. In fact, Caltech generously offered to also subsidize part of the campsite one year, so you might even be able to ask your point person at places if other departments might throw in extra subsidies to reduce costs to each individual.
1 week: This is the point you want the headcount complete, the car and driver numbers (just FYI, it will probably take the equivalent of one entire car to bring food and supplies, so plan for that), food restrictions, and equipment requests. There are many students at Berkeley who have equipment that might not be able to come, who should be more than willing to loan out things like tents and sleeping pads. The numbers might still fluctuate, but for the most part, this is the good concrete number. The things everyone needs are: a tent, a sleeping pad, and a sleeping bag. Things you will want in general are camp stoves, skillets, spatulas, and as many camp chairs as people can provide.
Couple days: Put drivers and passengers together, based on who wants to arrive late/leave early, etc… this is also a good time to collect contact information for all the drivers, in case you need to call them. Remember again: make sure that you have room for food and Sierra Conference supplies. This is also a good time to look in the Sierra Conference boxes in the department to see what you already have, and what you still need.
One day: This will be the stressful one. You need to collect all of the supplies that you are bringing, including the borrowed camping gear and Sierra Conference stuff from the department, and finally, food. For food, having a Costco membership is really useful at this juncture. We have found that having individuals take care of their own lunches works really well. So that means breakfast and dinners. If another school has many people, then you have one meal taken care of. Good meal suggestions are as follows:
Breakfast: Pancakes and oatmeal for breakfast. Get the kind of pancake mix that only requires water, and you are golden. Coffee and tea are also nice. French presses are really awesome for camping trips for coffee, so either get one for the conference or ask people for one.
Dinner: We tend to follow a 3 night pattern of BBQ-spaghetti-chili. This works really well, since spaghetti and chili do not need to be refrigerated. This means getting BBQ things for the first day (shrimp skewers have been a resounding success!), then fixins for the other days. Assume 1 1/3 servings per person, and always overestimate the amount of food you need. It is better to have extras to divvy up at the end of the conference.
Snacks: In case someone did not heed the directions to bring food, bring a few extra trail mix bars and the like. These are good whether people have lunches or not. It is also essential to have s'more ingredients, and hot chocolate is quite a hit at night when the temperature has gotten low.
Other: You should get propane that fits camp stoves (and want to make sure you have at least 2 camp stoves). If the site does not have potable water, you also need to get about 10 gallons of water per night. Wet naps are also very useful. So is biodegradable dishwashing liquid.
During: Take note of the last place you had cell signal, in case you need to call those people who are still arriving (or need to put up directions to a difficult-to-find campsite). Also note the general or grocery stores on the way. Something always seems to have been forgotten, so knowing the quickest way to get those things is really useful. You don't necessarily need to plan everything at this point, just have general information about the area so that people can decide the activity they want to do that day.
Afterward: Hopefully you have taken lots of pictures and had a great time. You are likely now several hundred dollars in the hole (hopefully split between a couple of people). Find out what departments are paying you, keep track of receipts so that you can contact everyone and split up what each person owes. This is best done based on the number of nights each person stays, meaning those staying three nights should pay slightly more than those who only stayed two nights. Having people be able to mail checks or Paypal has worked quite well at this stage, though you might have to email more than once.
There you are! Congratulations! You have learned everything you need to know about the Sierra conference! At this point, you now know people who have camping gear for next year, and probably know some new people from other California schools to bug for next year. Pat yourself on the back for a job well done.