I feel like a noob posting on this topic, as I am just getting started in the illustration industry, but thought I would share anyway.
1. Have a professional website
Get a website and name it with your first and last name, so people will remember your name. For example my website is erintaniguchi.com. Having an actual website and not a blog or tumblr is important as it makes you appear more professional. Post your best work in your portfolio; aim for 8-12 images. I first started with about 8 images and it has grown from there as I have created client work and personal work.
2. Create a mailing list
Build a mailing list of clients you would like to work for. I want to work in the editorial field, so I went to my local Chapters store and went through their magazine racks. I jotted down the names of the art directors and their contact info. I use a CRM to manage my mailing list; I use Insightly, which is free. I also recently discovered this list of art directors at Illustration Age. I would recommend to aim for about 50 contacts to start, and you can build from there.
3. Send out self promo emails
You will need to create self-promotional work, or pick your best 2-3 portfolio pieces. Send these images out, along with a short introduction of yourself and a link to your website. If you are lucky the art director may email you back acknowledging they will keep you in mind for future projects. I send out self promo emails every 2 months or so. Keep creating new work so you are always sending fresh work to the art directors.
4. Mail postcards
Another way of promoting yourself is to mail out postcards featuring your work. I tried this and didn't get any feedback, so I haven't done it since. I personally find email is more effective (plus it's free!). Printing and mailing postcards can also be quite expensive. But if you want to create a special mailer, this is a good option.
5. Join networking groups
Recently I have joined a business networking group that meets monthly. I find it helpful and inspiring to meet other entrepreneurs on the same path as yourself. This is also another way to meet potential clients.
6. Open online shops
There are many online shops to choose from now. I personally have a Society6 shop and a Threadless shop. Society6 I like because they do all the printing and packaging for you. Of course there is also Etsy if you want to personalize your packages and mail products yourself.
7. Have art shows
A great way to showcase your work is to see if any local coffee shops or restaurants want to show your work. These places are good because they are more likely to feature new artists. Shameless plug - I have an upcoming art show at The Wallflower in Vancouver in October!
8. Get into directories and annuals
Another way to reach art directors is through directories. Directories feature illustrators work and their contact info and are sent to art directors around the country. 3x3 has a curated directory which they offer to illustrators that enter their annual illustration contest. Be prepared to pay a pretty penny for these printed directories - a single page can cost several hundred dollars. The advantage is they are sent to thousands of art directors around the country. 3x3 also has an annual illustration contest where you can enter an illustration for possible publication in their annual. These contests are good to enter, as they get you in front of art directors that are judging the contest. You can also be picked to be featured in the 3x3 annual, which is additional exposure for your illustration work. There are several of these contests - 3x3, Applied Arts, Communication Arts, to name a few.
So those are my thoughts on marketing yourself, hope you found it helpful. Thanks for reading!