As opposed to discussing just the specific techniques used to target a precise niche market, I would like to start off with a few overarching thoughts. I will cover some basic, yet very useful principles to keep in mind for the bigger picture (Get it? 'Bigger picture,' like your career - hehe), and then supply a case study of how I have used them in my work.
I have put these three principles into practice to carve out a space for my work in the youth/artist niche market. They could loosely be applied to any niche market and to anyone working in the creative sector. From illustration, graphic design, to lettering, any 'creative' can apply these principles. Keeping them in the back of my mind, and using them as pseudo guidelines, has helped me stay happy with what I make.
Aesthetic Consistency
If the client asks for a child-like doodle, don't give them a photo-realistic photo etching. Obviously you want to create the right aesthetic on each individual project, which kind of goes without saying.
In the same way you want to create your aesthetic with your career. Figuring out, in general, what the images you make will look/feel like is extremely important. Now, I'm not suggesting you pigeonhole yourself and limiting your work to a specific style. I think showing a variety of styles is healthy for an illustrator, it's just that having an overall consistency is key.
Achieve an aesthetic, not just a style.
Clients, specifically art directors, don't like surprises. Like anyone, when a client comes to you for illustration, they want to know what they are going to get. So compiling a body of work that speaks to your aesthetic is extremely important.
In my own work I showcase a variety of illustrative styles, that doesn't mean that clients don't know what they are going to get, it means that I provide a variety of options. From a potential clients perspective it's extremely apparent what you're going to get from me; thoughtful, smart, often humorous and beautifully crafted images. This is because I have worked hard to carve out a niche.
Personality
Let your personality and attitude show in your work. This isn't something that should be forced or feel unnatural, it should feel authentic. Don't put fluffy bunnies on everything, just because you have a thing for fluffy bunnies. Expressing your personality can be sort of hard to explain, but a friends comment about a recent project I think nails it pretty well - He said "That's so you." My response, "Thanks."
Have fun thinking.
As noted in a previous paragraph, with my work the viewer is going to get a light hearted, smart, fun, witty, often humorous feel. And that's for good reason, I'm that type of fella. I have fun thinking. I like little funny, stupid, silly things that make you (or at least me) smile, and it shows throughout my work.
Subject Matter
Work on images you like looking at.
Though there may not always be options available, choosing a subject matter that you are interested in, or at least have working knowledge of, helps immensely. Working on images that you enjoy looking at really makes you happier. And when you're happy, at least for me, will lead to a better outcome.
Whenever possible try to take on projects where there is something, even something minute, to get excited about. This helps to find inspiration and really give it everything you've got. There is nothing worse than working on a project that completely bores you too death. Staying interested can take you a long way.
Prime Example
One specific prime example from my work that ties together all three of the principles that I touched on in this article is the illustration series I did a few years ago for Adidas Originals. From the aesthetic point of view the illustrations align with the modern and youth markets. The bright, bold color palette has a retro/vintage vibe that I often work within, so the aesthetics are consistent with the niche market.
With further investigation, the details showcase many simple, funny, playful aspects that reflect my personality. A polar bear wearing a scarf, a pink shark about to snag a surfer, a dog wearing a fugly Christmas sweater... that's funny stuff.
With the client being a top tier youth/athletic market brand that has a focus on forward thinking and innovative aesthetics it is a dream brand to work for. Not to mention Adidas Originals is a shoe brand that I have worn for years, my favorite being the retro High-tops, I have a huge interest in the subject.
Conclusion
I hope I have succeeded in offering up a few thoughts and examples that will help you in the long run, as well given some insight into a specific project within the youth and artist niche market.
