As you know, haiku is a short form of poetry with a distinct pattern: seventeen syllables split into three lines, composed first of five syllables, then seven, and then a final five syllables. Translations of traditional Japanese haikus often cannot replicate the syllabic pattern in English.
One way to know you are not writing in the usual style is to read many haikus—not too hard a task, as they are rather short. A better way to avoid the usual is to be mindful: the best haikus are about catching and evoking the imagery and feeling of a particular, fleeting moment. Often, it is an ordinary moment, but one we normally rush past. A good haiku causes us to stop and really think about that moment. For example, a famous Bashu haiku reads (and we lose the syllables in translation):
An old silent pond
A frog jumps into the pond—
Splash! Silence again.
This stops us and asks us to think about something so simple and seemingly banal that we have all probably experienced it: a frog jumping into water. But as we dwell on it, there is something deeply sensory and alive in that movement that contrasts to the stillness of the "old pond." Perhaps we need to pay more attention to small moments of disruption? Perhaps there is something profound in the splash?
If you write what comes up for you as you observe mindfully, you will write from your own soul and not what everyone else says. That might involve peeling the onion—writing some cliches or lines that feel "wrong" and then getting down to the essence of what you really want to say.
The keys to writing haiku are first to find an image—something you can convey through any of the five senses of sight, sound, taste, touch, or smell—and describe an aspect of it in the first two lines. Usually you would focus on a detail. Then the emphasis—the point of what you are trying to say—falls on the third line. In the haiku above, we get the quickest sketch of the old pond and the frog jumping in, but the emphasis falls on the "splash," the moment of disruption.
To get into the mindset to write haiku, it is important to be still and really focus fully, with alert attention, on something around you, so that you experience it deeply, with enhanced mindfulness. That 'something' can be anything: to write a nature haiku, you would want to sit outside, take everything in, discern what is capturing your attention, jot down notes that describe it, and then address how it makes you feel. At that point, you are probably ready to write. But you don't have to be outside: the way the light shines on a piece of plastic on a table can conjure images of water, for example. Often, because it is short, we are corralled into writing haiku too quickly, without time to still our mind and really grasp the essence of an object.
You might want to look at Natalie Goldberg's book on haiku, called Three Simple Lines.