Step 1: Get near to the cap on tiles with improvements built on. Ideally, 3 or less tiles left.

Step 2: "Absorb" a nearby improvement into your city. This counts, for some strange reason, as tiles with stuff built on them for your city. This will most likely drop your available tiles by 4 - bringing you BELOW 0, to a negative number of available tiles. At this point, you will be able to build as many improvements as you like. The cap is gone. Note that your city's fence will most likely bork up and possibly crash the game if you place stuff in certain formations.

Note: I have seen AI players doing this as well. I currently own a city I captured from another civ that came with -8 tiles available.