If your good at the human figure, you might want to narrow that specialty down into further subsets for your portfolio (as well as strengthen weaker areas in your own time). Ex strength in clothed figures, but need work on portraiture or anatomical images. Really this all depends on what you want to get into as the illustration field is vast in options.
However do have others, especially artists, take a look at your work when crating for your portfolio. If you plan to make art for others, it is frankly the opinion of others that decides your art’s meaning and worth (hard to hear, I know, but true). You may have in your mind what work is your best, and it is a worthy opinion to keep, but others opinions may tell otherwise. Consider it a critique and work with it, understand where it comes from and why.
Making the connection to what is an in demand is also another tricky part.
Illustration opportunities: whatever you are looking at, its all about what the company needs. Research patterns of their work and try some of your own. Contact them with a postcard or contact sheet of your work, when you feel ready to strut your stuff to them. Don’t over send, just send stuff every 60–90 days if you really like a place. Networking helps more than self promotion a lot of the times, so get to events too and get to know people! In the end, you will go under the radar for a while unless you are exceptionally skilled, expect that and continue to create personal work and try to get more work.
When jobs get posted: know you may have to make example pieces on the fly if you have no current examples. I know a fellow who painted up 10 boats and horse in less than a week to land a job with that focus. He didn’t get that job, but now he has those example for next time, and they did come in handy.
Self promotion: gaining a large audience is another way to have the work come to you. If enough people like your stuff, you get around. And those who pay for illustrations will either see you often enough to notice, reach out to you for your specialty or style, or when you contact them- they might consider your potential and hang on to your contact.
But to get a large following, you do have to play to the trends. An example would be, you are very good at illustrating clothed figures. Perhaps produce some fashion illustrations, retro ad illustrations, pinups maybe? You can do even more to please your fans, but that’ all up to you (plenty of info on the web). This all goes back to what facet of the illustration field garners your interest and what parts you are just filling in to be a bit rounded or experiment.
My final line of advice: Don’t do everything. You will have more success if your portfolio has a focus. If you are all over the place, those who look at you will have trouble deciphering your strengths. They don’t want a mixed bag, they want a master of insert need.
A hard truth, but think of how many artist are in the world. Your competition are not just other new artists, but the older fellows who are already working with companies. You need to bring to the table something they can’t do, or hope to be the one to replace them as they move to different focuses.
With experience and many clients/jobs, it is only then can you comfortably be a jack-of-all-trades as you have dabbled and successfully made work in various focuses or fields of illustration.