People have five senses, but we only really use two of them while driving: our sight and hearing. You might smell or even taste burning rubber or brake dust, but if you ever use your sense of touch for more than grasping the steering wheel, things have gone horribly wrong!
So when communicating with fellow motorists, you have two tools at your disposal: your lights and horn. That's right: Your headlight can be used for more than illuminating the roadway at night.
Motorcycle safety experts have long advocated running your high beam during daylight hours. Their thinking is, the brighter the light, the better you'll be seen. And if you've got a sealed-beam headlamp (or an older 6-volt system), by all means do so. But if you've got a late-model bike with a halogen bulb (or two), you might want to reconsider that. Headlights are so bright nowadays that they can be blinding even before the sun sets. If you're not sure, ride toward one of your buddies and see what he thinks, or set your bike on its stand, walk down the road a ways and have a look.
Assuming your headlight is bright enough, running your low beam during the day lets you use your high beam to get other motorists' attention. Think of it as a visual horn. Riding with your high beam on all the time is akin to typing in capital letters: IT'S CONSIDERED SHOUTING! Don't be the boy who cried wolf; save your high beam for when you really need it.
Your brake light can also be used to communicate. If a driver is following too closely, tapping the lever or pedal just enough to illuminate it should cause him to back off. Likewise, if you're sitting at a stoplight and aren't sure a motorist approaching from behind sees you, flashing your brake light will alert him to your presence. Using your brake light in conjunction with your turn signals not only lets surrounding drivers know you're planning to turn, but (in the absence of hazard lights) can also communicate that there's danger ahead, such as debris in the road, an accident or an emergency vehicle approaching.
Which brings me to my next point: Perception is reality. If you flash your high beam, tap your brake light or turn on your turn signal, you need to consider how other motorists may interpret that. The California Department of Motor Vehicles suggests signaling 100 feet before turning, but if you signal too early, a motorist might misunderstand what you mean. For example, if there's a shopping center in the vicinity of an intersection, it's better to wait before signaling so a motorist won't think you're turning where you're not and pull out in front of you. On the flipside, forgetting to turn off your signal after you've completed your turn is inviting someone to cross your path. Make fun of Harleys all you want, but they have self-canceling turn signals for a reason.
On a related note, you can flash your high beam to get the attention of a motorist waiting to turn left, but it's best to leave it on until after you've passed him. A quick flash might be interpreted as you saying, "Go ahead." You want him to know you mean, "DON'T YOU DARE MOVE!" If at all possible, make eye contact with the driver so you can read his intentions. That works even if you're wearing a tinted face shield or sunglasses.
As feeble as horns are on most motorcycles, I seldom use mine, and when I do it's more often for bicyclists, pedestrians and animals who might actually hear it. Considering how many people are driving around with their Bluetooth headsets on and/or their car stereos cranked, they probably won't hear you anyway! I try not to honk when I get upset, either. If a motorist violates my right of way or moves over on me, I simply brake, take a deep breath and continue on my way. If you expect the worst, you'll hardly ever be surprised. And losing your cool on a motorcycle just makes you an easy target.