Review IO at the ugrad level. Read up on antitrust or whatever you'll be doing, both recent law (eg check out things like SCOTUSblog) and Econ. Read some expert testimonies - ask your new boss or colleagues. Read some legal complaints and decisions. Brush up on Excel. Practice communicating wth people who don't know economics.
Also, a big part of success is mental fortitude. Find the most boring, menial tasks you can think of and then do it for 60 hours a week. Take an old paper you wrote and at 10 PM, decide that you need to redo the format of all your citations and it has to be done by the morning. If a friend of yours has ever created a powerpoint presentation for work, ask him to print it, scan it, print it, scan in, print it, scan it and email it to you. Then go through the powerpoint presentation and put together a memo containing 1) detailed information of the products his company sells, and 2) his company's profit and loss for the past 5 years. Then hand your memo to a person who knows nothing about anything and if that person doesn't immediately understand the product and all of its financial implications, you're a bad associate.