Ideally, have a legal support team in place, with lawyers and legal workers trained to understand the principles of solidarity. At the very least, know some lawyers you can call on for emergency help.
Inform people. Trainings and preparations should include basic legal and jail information. Legal briefings can be offered before the action. Handouts with basic information and phone numbers can be available at the action.
Know what your solidarity strategy is, and include information about it in trainings and preparations.
Know who your political allies are that you can call on for support. Unions, NGOs, sympathetic politicians, Green Party members, religious groups and progressive mayors may not be willing to go out on the streets with you, but are often willing to help get people out of jail, or to pressure authorities to provide decent treatment. Progressive journalists, and civil liberties groups and legal associations are often your best 'safety net' in terms of generating immediate public debate and support for your cause.
You need to be aware of how your actions and strategies can be used against you by authorities and the media. If your group can be painted as being 'extreme', then it may make it less likely that you can generate support and solidarity. It can also provide authorities with the opportunity to increase repression of your group. In the Post 911 environment, use of emotionally laden tags like 'terrorist' and 'extremist' can be used by authoroties and commentators in order to drive a wedge between specific activists and the general public. In taking on specific actions or protest activity it is always worth considering how that action could 'play out' in the media or whether it might alienate potential supporters.