If people are going into a known arrestable situation, know as much of people's info as they're comfortable giving - including real full name, arrest history (not just activism related), outstanding warrants, responsibilities they need covered if arrested, emergency contacts. If people are unsure whether they may be putting themselves in an arrestable situation, ask them to consider these things beforehand anyway and make it clear that they should get this information to a legal support person as soon as possible.
Arrange ahead of time, and let ALL of your affinity group members know, a local number that accepts collect calls from jail where you can be reached, or that you will be checking regularly and frequently.
Know or find out people's medical info: do they suffer from asthma, heart problems, allergies; do they require prescription medications; and record their doctor's name and phone number.
Have access to people's IDs, bail money (or sources of bail money - friends, parents, etc.).