Don't forget to pack these essentials!
1. Comfortable shoes
You'll be doing a lot of walking, and likely won't have time or a place to sit down and rest, so make sure you're wearing shoes that won't leave your feet in pain at the end of the day.
2. Cash! (In small denominations)
The majority, if not all, of the circles will not be able to accept credit or debit cards, so make sure you have enough cash with you to get everything you want. Also, many circles will not have change for large bills, so bring your cash in small denominations. (1000 yen bills and coins are probably best.)
3. List of booths with locations and map
If you can get your hands on the Comiket catalog before the event, make a list of booths you want to visit and their locations. The Comiket catalog is the size of a phone book, and heavy. It will be much easier to carry only a map and list of booths you're interested in.
In the summer, this is absolutely essential!! August in Tokyo is already hot, but if you add the crowds of Comiket it becomes almost unbearable. Make sure to drink a lot during the day to avoid becoming dehydrated. In the winter, water isn't necessary, but it's never a bad idea to bring a bottle along.
5. Light, breathable clothing (For Summer Comiket)
You'll want to be a cool and comfortable as possible.
6. A light jacket (For Winter Comiket)
In December Tokyo is cold; however, the crowds of Comiket will heat things up quickly. Bring a jacket, but don't bring anything too bulky.
7. A bag to carry all your purchases in
You'll have to commute back to your hotel at the end of the day, so make sure you bring a bag big enough to carry all your purchases with you.
You'll probably want to take many pictures!
9. Cell phone
If you have a cell phone that will work in Japan, bring it in case you get separated from friends. However, due to the large number of people who attend the event, cell phone service may not always be reliable. It's also a good idea to establish a meeting point and time should the crowds push you apart.