Bob Woodward has written up an amazing description of the security practices instituted by Mark Felt during his Deep Throat period. Felt had been a G-Man for years, and had shadowed various suspected Soviet agents, so he knew how the Fed worked when it wanted to watch you, and devised a plan that would keep his deeds secret from his employer.
If you keep the drapes in your apartment closed, open them and that could signal me, he said. I could check each day or have them checked, and if they were open we could meet that night at a designated place. I liked to let the light in at times, I explained.
We needed another signal, he said, indicating that he could check my apartment regularly. He never explained how he could do this.
Feeling under some pressure, I said that I had a red cloth flag, less than a foot square — the kind used as warnings on long truck loads — that a girlfriend had found on the street. She had stuck it in an empty flowerpot on my apartment balcony.
Felt and I agreed that I would move the flowerpot with the flag, which usually was in the front near the railing, to the rear of the balcony if I urgently needed a meeting.
(via Schneier)