Meetings

When organising a private meeting, if you cannot send details to all involved in ways that will not be intercepted always try to agree on meeting in one location near to the meeting place. You can then direct people to the correct location as they arrive. By keeping the location of a private meeting limited, you lessen the likelihood of the location being surveilled.
If meeting in the home or building of another person or organisation do not make a phone call from their phone to a number that is identified with you, or from a public phone box near to that building.
If the people going to a private meeting are likely to have mobile phones, ask them to turn them off before travelling to the meeting place (if all the mobile phones of a groups of people are in the same cell at the same time on the same day, it can be assumed that you have had a meeting).
If you require a private meeting place, do not keep using the same one. Alternate it as much as possible. Also, if you meet in a public place, pick somewhere with a high level of background noise, and with as many obstacles or partitions around the point where you meet, to prevent your conversations being overheard.
If you must pay for something whilst having a meeting, use cash. Or, if you cannot, get one person to pay. In this way you will not generate paper trails linking you together.
Meeting in public spaces, streets, in parks, or on public transport is not a good idea - many of these areas are surveilled by CCTV. But bars, cafes and restaurants tend not have their CCTV systems linked to a central control room, and what CCTV systems are installed are concentrated around the till.