One of the first steps of my design process is to research my client's competition. After all, what I want to accomplish is to not only attract customers, but to differentiate my client from their competitors. If I can make them look the best, the potential customer will pick them over the other guy.
Many times in this research, certain design trends for an industry start to emerge. My work is easier this way, as my client's competitors seem to be all copying themselves. So what do I do? As long as it still communicates the marketing objective, I try to do the opposite. Different colors, images, layouts, typography...anything to separate my client from the competitors.
Because of these common design themes within an industry, sometimes a client will want to do what all their competition has done: copy these designs. In most cases this is the worst thing you could do, and the client must be persuaded from following the same trap that their competition fell into themselves. Standing out from the crowd can make your target audience pay more attention to you and your product or service.
In competitor research, it's also important to see what they've done right. What phrases and messages have they created that really work? What have done with the design that communicates? You can still use this information without copying the look. For example, I did a website for a skin care salon. Most featured soft images of a woman in a spa setting. This definitely works and isn't the wrong thing to do. But, to stand out, I choose a more striking image of a woman laying upside down with rose petals surrounding her. This was enough to use what worked, but did something different to stand out.
What ever you do, do your research. The worst thing you could do besides not knowing your target market is not to know your competition.