In wireless sensor networks, the many-to-one data communication pattern induces high collision losses as multiple transmissions cause contention and interference along the paths from sources to the sink. This paper proposes a low-overhead MAC layer solution to address the high contention problem to improve system throughput and reduce energy consumption. Periods of bursty transmissions with reduced contention from neighboring nodes are exploited to efficiently clear up backlogged queues and improve the performance of CSMA. Through analytical modeling we characterize the expected performance improvement and show that it conforms to the simulation results. Using extensive simulations on ns-2 and experiments on the 49 node sensor network testbed (Kansei) running TinyOS, we evaluate the performance of our solution.