MTTs vs. ring games

I can't believe that the two differ so much. My MTT game is pretty solid by my standards. But once iin a while if I'm short on time, I sit in on a ring game and invariably, I lose my ass.
What do you think are the main differences? In general what are a tourney players main mistakes when they switch to ring games?

PS. Ive read many MTT books and I know one of the answers to the above question is to read 'Harrington on Cash Games'. Excepting that(which IS on my to do list), is there anything else?

Thanks,
FITEMAN