As a few mentioned, keeping your broodnests open is the best way to suppress swarming. It must be done early enough to be effective, before they decide to swarm ;) which means looking in 'at least' twice a month. Someone should really write a book on KYBO (keeping your Broodnests open) because it is basic beekeeping, albeit known by many different names, which likely cause great confusion (so it goes). It must be practiced and perfected to be effective. Too many beeks simply add another box when bees 'seem crowded' but that never works, whether for expanding brood nest or expecting honey in return. Bees need a bit of coaxing, not just an 'empty' box.
Open up your colonies and locate the brood nest (bring an extra box with frames) remove some brood frames (1 or two or three or five depending on number of bees/boxes/hive type) from each box, placing them into 'new' box, each separated by an empty frame. Place empty frames where you removed brood from, keeping just 'one' empty frame between each frame with brood. Try to keep brood together as you stack them back up. That's KYBO. Hope this makes sense.
While it 'might' slow down the tendency to swarm, splitting your colonies gives you 'more' hives, not necessarily more bees, and if that is what you want then you should make splits, there is much written on the various methods.
However, KYBO makes your existing hives stronger 'and' less prone to swarm and if done in a timely and methodical manner will outperform any other method to prevent swarming. But............... sometimes they just swarm no matter what you do. But don't be sad about losing one now and then, that's like being sad for having sex :-D. if you're a bee.
thomas
