I recommend making use of hashtags to help people find your posts (particularly useful if you're publishing blog posts). People can set up searches that will display any tweet that features a keyword that they're searching for, for example #illustration or #designjobs.
Just try and think "what are the most logical hashtags to use for my tweet", for example #bloggingtips #graphicdesign (or #graphic #design, which may reach a wider audience since it's two keywords).
You could also take a look at what's trending and tweet about those (if it's something you're interested in).
My followers aren't particularly high but I do notice that the count increases after tweeting using hashtag keywords.