The Magellan AO system combines a pyramid wavefront sensor and high-order adaptive secondary mirror, and will see first light on the Magellan Clay telescope in November 2012. With a 24 cm projected actuator pitch, this powerful system will enable good correction in the optical (0.5 to 1 μm). Realistic laboratory testing has produced Strehl ratios greater than 40% in i' (0.765 μm) on bright simulated stars. On fainter stars our visible AO camera, VisAO, will work in the partially corrected regime with only short moments of good correction. We have developed a form of lucky imaging, called real time frame selection, which uses a fast shutter to block moments of bad correction, and quickly opens the shutter when the correction is good, enabling long integrations on a conventional CCD while maximizing Strehl ratio and resolution. The decision to open or shut is currently based on reconstructed WFS telemetry. Here we report on our implementation and testing of this technique in the Arcetri test tower in Florence, Italy, where we showed that long exposure i' Strehl could be improved from 16% to 26% when the selection threshold was set to the best 10% of instantaneous Strehl.