The explosion of mobile traffic volume has led to dense deployment of IEEE 802.11 WLANs. As a consequence, periodic beacon transmissions can overwhelm the air-time, leading to significant air-time depletion for data transmissions. In this work, we develop an opportunistic air-time sharing scheme, named BeaconRider, that facilitates simultaneous data and beacon transmissions aimed at improving spectrum efficiency in dense network environments. The proposed method works for downlink communication and allows access points (APs) to coordinate with each other in a distributed manner to exploit opportunities provided by the capture effect. Our protocol is backward compatible with legacy 802.11 APs. Through experiments with a prototype implementation using off-the-shelf IEEE 802.11n dongles as well as extensive ns-3 simulation, we show that the proposed method achieves substantial performance gains that increase with the number of APs.