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how do you stay under the radar?
I dunno -- don't step on anyone's toes by advertising in the same places. The more you shout about your product the more potential competitors will notice. The catch22 is that most often you need to get the info out there to gain customers.
People talking about how well they are doing is a sure way to gather new competitors fast.
Don't worry about it, just go for it while it lasts and try to diversify in case marketshare loss starts to become an issue.
However, stay static and it's likely you'll be taken over in two years time I'd say.
As already said, if it's a booming industry/vertical then keep an eye on competition...if it's a relatively stable industry then you'll probably be ok.