I'll throw out the MyFICO forum. http://ficoforums.myfico.com They have all kinds of advice about boosting/repairing scores. Everybody's credit situation is different but from what I remember, credit cards used responsibly offer the greatest opportunity to build or repair your score (generally speaking).

I'd recommend going with a credit card that has that free monthly credit score feature so you don't have to pay to monitor your progress. In addition to Citibank, Discover also offers it on (AFAIK) all their cards.