I know of a good number of guys who got caught up because they overflew an AD (due to some pretty crappy stuff). Anyhow, we have had a LOT of chats with the feds recently about this sort of stuff, and it's pretty simple.
First, if they contact you to find stuff out, they already know more than enough to hurt you. They don't go fishing by sending you a letter or chatting on the phone. Second, if they request an informal meeting, it is just that, and the last thing they want to see is a lawyer (they don't bring thiers--don't bring yours). For the most part, they have a lot of leverage in the punishment, and that means that cooperation is a HUGE help. For the most part, they do the informal meeting and a punishment is decided.
If you're decent to them, they'll be decent to you (duh). The most common thing our guys do is you hand them your certificate, and ask them to keep it in thier desk for a while. Next to that, our FSDO required our guys to teach a class on ADs to all the other employees of the FBO and thier students (with the feds there). Kinda childish really, but at least the feds got to learn a lot more about us and the way we do things, and we got to learn a LOT more about the enforcement process, as well as some seriously wacked stuff about Mx regs... I'm still trying to figure out how an inoperative nav light makes an aircraft un-dispatcheable during the day--without an A&Ps deferment. But, c'est la vie.
Dan