Increased NAND flash memory density has come at the cost of lifetime reductions. Flash lifetime can be extended by relaxing internal data retention time, the duration for which a flash cell correctly holds data. Such relaxation cannot be exposed externally to avoid altering the expected data integrity property of a flash device. Reliability mechanisms, most prominently refresh, restore the duration of data integrity, but greatly reduce the lifetime improvements from retention time relaxation by performing a large number of write operations. We find that retention time relaxation can be achieved more efficiently by exploiting heterogeneity in write-hotness, i.e., the frequency at which each page is written. We propose WARM, a write-hotness aware retention management policy for flash memory, which identifies and physically groups together write-hot data within the flash device, allowing the flash controller to selectively perform retention time relaxation with little cost. When applied alone, WARM improves overall flash lifetime by an average of 3.24× over a conventional management policy without refresh, across a variety of real I/O workload traces. When WARM is applied together with an adaptive refresh mechanism, the average lifetime improves by 12.9×, 1.21× over adaptive refresh alone.