Bring one pair of thick, comfortable socks per day. Shoes are not allowed in the zen center past the front foyer. You'll be spending most of the day in socks. The floors are wooden. You'll see people going barefoot in the zendo. This is traditional for monks. If that suits you, that's fine. It's not more "holy" to go barefoot.
Pack comfortable, loose, but "well-ordered" clothing in dark, muted hues which is comfortable to sit in.The idea here is two-fold. On the one hand, you are going to have enough discomfort to deal with because of sitting still for hours and hours. If you haven't done this before, you know not what you are going to encounter. You do not want to compound whatever that ends up being with clothes that bind, pinch or otherwise add to bodily discomfort.
On the other hand, you do not want to distract other people. Sesshin is a group activity, be mindful of others and choose clothing that is uninteresting yet ordered and uniform. It should be neither too casual (work-out clothes) nor too formal (business attire). Blue denin jeans are okay as long as they are not excessively sexy and/or form-fitting. If you have a hot body, that's fine, just don't guild the lily.
Black is the color of Soto zen students, brown is the color of zen teachers. There is no prohibition against wearing brown, just know this when making your choices. I usually sit sesshin in black jeans and a black polo shirt, but I did not purchase these for that purpose.
There will be work periods, part of which may involve working outside or using cleaners that might spill on your clothes. If the clothing you are sitting in is not hardy enough to withstand a minimal amount of abuse, bring some work clothes. You'll have time to change in and out of them.
Bring a hand towel. Practice leaving a light garbage foot-print. Again, always be mindful of the group, frequent hand-washing is the norm so we don't pass around microbes excessively.
Bring a bath towel and bedding. Staying at the zen center is analogous more to being a houseguest than a hotel patron. There will be a place for you to sleep, but you'll want to be self-sufficient and leave it as you found it.
Shower before the sesshin starts. You won't be showering or shaving during sesshin unless something unexpected happens. That's okay, we're sitting most of the time and you'll be happy to use the half-hour you'd be showering for sleeping. You'll have a chance to shower when it is over.
Forget your watch, forget the comfort of always knowing what's coming next. Pay attention to the group, ask questions of the practice leader (sometimes called the ino - "EEno") if you're confused, but if you follow the pack and listen for the bells you'll be fine. A watch is just going to distract you.
Forget about reading and writing. You'll have time for neither, so you don't need books along and this won't be the time to start that journal you've been meaning to get to.
Do not look people in the eye. This is distracting. Sesshin is a time to "collect the mind," that's what everyone is doing. That's hard enough to do without having to return, understand, or digest eye contact. This is the social contract of sesshin.
Do not speak to anyone. See above. Of course, if you have to talk to someone for an operative reason, that is, to do something, get something, or to avoid some problem, do so. Don't be a rigid idiot and cause some problem for yourself and/or someone else because you're keeping silence, but make no social conversation.
Do not worry about things you forget or foul-up. Everyone does it. When you've messed up, know that the rest of the group is smiling inwardly because it wasn't them. You just brought a little bit of joy to the entire sangha. The teacher will notice ONLY how you recover from the error or ommission. The teacher couldn't care less about the infraction itself, once it is over it is as relevant to the teacher as the fact that you soiled your diapers as a baby decades ago. The teacher is interested in how you bring yourself back to the present moment.
So, if you've dropped a serving spoon and flung oatmeal on the zendo floor, the teacher is only interested that you attend to the spill with mindful dignity. It is much better to fling the oatmeal and clean it up in a manner that seems as natural as your next breath than to be contrite and embarrassed.
Everyone in that room has done something like whatever error and omission you've made, including the most senior and learned teacher. They are not annoyed with you, they are happy it wasn't them this time.
Expect to fall in love with someone, and to be completely disgusted with someone. This is what the mind does when left to it's own devices. Just notice it and return to your breath. It is meaningless other than to illustrate vividly how our minds can shape our perceptions.
Do not try to meditate. Just sit and follow your breath. It may help you to count your breaths up to ten and back down again. It may help to notice the cool sensation at the back of your throat when inhaling and the warm sensation there with exhaling. It may help to attend to the very tip of your nose and notice the sensation of air passing back and forth. That's what to attend to. Everything else that passes through your mind should be released as soon as you realize you've grasped it. There are lots of metaphors for this, some people see discursive thoughts as clouds passing by, others (myself among them) use a train metaphor. I see the train and simply do not hop on, watching it pull out of the "station."
At some point you'll stop even needing to redirect your mind to your breath, or so I'm told. I'm not there yet.
Chiding yourself for getting lost in discursive thought is another discursive thought. Get off that train too. Return to your operative thoughts, i.e., what you are doing. You are breathing. Think about breathing, or more correctly, be your breath.
During rest periods, rest. This goes for eating and working, too. Do what you're doing. When you're working, be the work. When you're eating, be eating.
Chant with your ears. Sesshin is group activity, listen for the tone, rhythm, modulation and volume of the group, blend your voice into it. If you get lost, stop chanting until you've found your way again.
Find someone close to you that knows what they're doing. Watch them, turn to the page in the chant book that they turn to. Picks things up when they pick them up, put them down when they put them down. Get up when they get up, sit down when they sit down. Do not try to figure out for yourself what to do, follow along.
Stay in ear-shot of the bell.
Keep your hands in shashu. Make a loose fist with your left hand, but put your thumb in the middle of it, wrap your right hand around it, place both hands over your solar plexus. Doing so quiets your mind as well as everyone else's. The only time this is the incorrect posture is when you are in gassho (hands with palms together, as in prayer, fingertips even with your nostrils) or when you are in meditation (when you use the universal mudra) or sitting attentively on your mat in the zendo, as during meals when you aren't eating. Don't try to remember all that, do what others are doing. It is perfectly okay, expected even, to watch what others do, it's looking them in the eye that you try to avoid.
Enjoy yourself People will look sort of grim, but that's because they are attending to their inward lives and not paying attention to their facial countenance. This is a precious time to focus on your practice, you are most fortunate to have the opportunity. It will be over soon, don't waste a precious moment in pursuit of anything else.
Expect to break the rules and foul things up. Practice is noticing that and returning to the moment. Practice is not ANYTHING ELSE.