Creamegg, I'm not a pilot and I may stand to be corrected, but generally, when in the cruise, the aircraft will be slowed down when encountering moderate to severe turbulence as a means of further reducing any potential stress on the airframe. Otherwise, as said above, today's aircraft can take a great amount of punishment. It actually feels a lot worse than it is. Finally, as also mentioned by MrSoft, sitting anywhere near the rear of an aircraft (any aircraft, not just the B757) will accentuate the effect of turbulence. Yes, it can be very unpleasant (especially if it goes on for lengthy periods) but rest assured that you are safe.