Shorter version for print
Dispatch is basically a system to collect, process and disseminate accurate, reliable and up-to-date reports on protests and actions. There are many different ways and techniques to do this but the main principles remain the same. There are three main elements/processes that make up Dispatch: incoming info (reports, phone calls etc.), processing (confirming, double-checking and structuring the data) and outgoing info (timelines, articles etc.).
In addition to the obvious things such as technical and logistic arrangements, preparation means Dispatchers should know enough in advance about the issues/actions/campaigns they will be reporting on so as to avoid embarrassing mistakes. Maps and large sheets to stick on the wall are some of the obvious things you would need. Write down a detailed timeline of the dispatch subject, including all planned actions and events. Find out times and places of meetings at the convergence centre(s), plenary sessions, info meetings etc. The most important thing is probably contacts: in addition to publicising your dispatch phone numbers, you should have a list of the contact details of relevant groups and activists, as these would be your vital source of information/confirmation. Finally, make yourself familiar with IRC, SiLC and the like in advance, as these are usually used for Indymedia Dispatch.
There are normally two ways by which dispatchers get information: passive and active. Passive is when you just wait to receive information via phone calls and/or other channels (IRC, for example). Always ask your source what exactly they witnessed, when and where. Ask them whether they actually saw what they are reporting themselves. Also, what else is going on right now in that place, and what is likely to happen next. If the source did not witness him/herself what s/he just reported, always treat the information with utmost care. Active is when you call people you know and trust (IMCers, medics, legals observers etc.), who are at the place where something is happening. Ask them whether they can confirm unconfirmed information or ask them to go to the place and check. When they report back, ask accurately whether they saw themselves what they are reporting or are reporting a rumour/something they heard. Other 'active' ways of getting reliable information include watching webcams; calling official bodies (hospitals, police, press offices etc. ); keeping an eye on the official websites of the campaigns involved and even the mainstream media.