It's just like fishing. Every little thing helps, including some luck. Old comb helps. Lemongrass oil helps. The "swarm lures" seem to work the same (and smell the same) as the lemongrass essential oil. Some old queen juce (dump your old queens in a jar of alcohol) also seems to help. Old equipment helps. Setting it up high (at least 10 feet or more) seems to help too.
I seem to get about one swarm for every ten boxes I set out. But, just like fishing, location is everything. If you find a place you're getting swarms, put more there. If you find you're not getting swarms, move the bait hives somewhere else.