Learn Acidus Novuus! Seriously, if you only learn one thing, make it Acidus Novuus. It is one of the single most useful and powerful, and at the same time, easiest moves in all of mentalism and magic.
It's even explained on the Stigmata DVD, it's also on Richard Osterlind's Mind Mysteries Vol. 1 along with His Perfected Center Tear, it's used in the watch routine.
Learn a good card force, and learn a good stack. Si Stebbins will work, but Osterlind's Breakthrough system is better.
If you are working standup or stage, learn a "Major" effect, which is a method of gaining large amounts of information from the audience, like a Q&A act, or a dunninger-esque audience reading act. Both Bob Cassidy and Richard Osterlind have lots of work on this. This is what will make you a legend.
If you are looking to buy something, buy The Mind Mysteries dvds, at least #1 and #2. In just these 2 dvds, you will learn the center tear, acidus novuus, a Q&A method, one of the best card stacks on the market, and the radar deck, which is something I keep on me all the time, amongst a whole bunch of other stuff. Seriously one of the best values out there.