They never actually leave. They’ll just join the other 370 former members of congress who are currently lobbyists or advisors.
http://www.opensecrets.org/revolving/top.php?display=Z
BLUF - You can get more as a lobbyist if your resume doesn't include a failure to successfully run for reelection.
Therefore it is better to bail prior to the electioneering season than to be thrown out of office in January. Just follow the money, something that all Congress Critters are famous for.