Other than stepping outside and trying to hail one, you should probably get the GetTaxi app for your smartphone. As all great things (or so we like to think), it was started by a bunch of Israelis, and lets you easily order a taxi without having to wait outside for a single moment, and you can even pay with your credit card.
That said, getting a taxi on any of Tel Aviv’s main streets normally shouldn’t be too much of a problem anyway.