Use Your Paper and Pencil; Not Your Head

Today, I met a prospective client in California. Super smart guy. I noticed, however, him doing everything in his head. The Analytical Reasoning (a.k.a. Logic Games) question was asking for a min/max list. He clearly knew what the question was asking for and he also knew exactly how to derive at the correct answer. However, he didn't get the correct answer. All because he didn't keep track of which variables he already used (and which ones he had not yet used).

He could have avoided this careless and silly mistake by simply remembering to write out his work. 

There's nothing wrong with writing things out. It doesn't mean you're any less smarter than the person who can do it all in his head. The LSAT isn't measuring how fast you can deduce and calculate something in your head per se. It's a multiple choice test that measures your accuracy + speed. Avoid silly mistakes. Take the 2 nano-seconds to write out your work.