Be sure to check your opponent's build before you itemize against them!
Kind of a PSA, not really that big a deal. I just played Teemo into Gnar earlier today, and the Gnar decided he was gonna be slick and get some early MR. I started, however, by building into Frozen Mallet and went for more of an AD/dueling build, and beat his ass all game because he wasted money in the lane and thus got put far behind.
Just try not to waste your gold, especially if you're behind, by building the wrong defensive options.
instead of the full tank 
would still help you a lot laning vs ad teemo