Swarm Cells

Preventing (and utilizing) swarms is a BIG topic, one that is probably best explored through conversations with local beekeepers.
Having said that: one very small but practical aspect of monitoring the swarm impulse in your hive is to watch for swarm cells (also called "Queen cups") on frames in your brood boxes. The cup shaped cells on the bottom of this frame indicate that my bees are thinking about raising another queen to take over in the existing colony, after a bunch of the bees take off with the older queen in a swarm event.
I've heard that some beekeepers will scrape these cells off; my personal inclination is to let the bees do their thing but try to encourage them to stick around by giving them more space to expand in to (by 'checkerboarding' blank and brood frames, adding an extra brood box, etc.) and, as a backup, leaving an extra Western in my yard scented with lemongrass oil. Lemongrass oil (and the smell of wax, if you have drawn comb) can entice a swarm to move into this kind of 'swarm trap.'
Remember: swarms aren't bad! Except maybe for you... (and your neighbor, if they don't like bees). Swarming is the honeybee's natural method of propagation. I've heard it described as "the ultimate act of self-sacrifice": the queen and a retinue of workers leave behind a healthy, functional home to head out into the great blue yonder and try their luck. Feral colonies of bees that survive are often- by default- varroa-resistant, and provide a useful asset to the honeybee gene pool.