Flash memories are considered a competitive alternative to rotating disks as non-volatile data storage for database management systems. However, even if the Flash Translation Layer - or FTL - allows both technologies to share the same block interface, they have different preferred access patterns. Database management systems could potentially benefit from flash memories as they provide fast random access for read operations although random writes are generally not as efficient as sequential writes. In this paper, we propose a simple data placement algorithm designed for flash memories, to reorganize inefficient random writes in a quasi-sequential access pattern. This access pattern is first established encouraging for a subset of flash devices by identifying a strong correlation between spatial locality and write performances, with a distance being defined to quantify this effect. This design is then validated by a formalization with a mathematical model, along with experimental results. With this optimization, random write potentially become as efficient as sequential write, improving random write speed by up to two orders of magnitude.