Cilantro is tricky because several factors can cause it to bolt. Avoid transplanting for this reason, and avoid hot conditions as well as too much moisture. It does best in light, well-drained soil in partial shade, in relatively dry conditions. This is easy to achieve beneath a cloche in winter, where cilantro will thrive. Once it blooms, the seeds ripen suddenly, in only a couple of days, so care should be taken to prevent self sowing or simply losing those useful seeds.