I feel like this thread is made for me - I cut my own hair, I've been growing it out, and have been self-trimming for awhile, but in general I only actually get a cut in a salon maybe once a year. The rest of the time I go it alone using my sewing shears since they're the sharpest. What I do is I section off my hair, and then twist the sections with my fingers, and then just snip off the very very end of the section. I do this all over my head, or usually just the areas I want shorter. The smaller the twists, the less choppy it looks. I take off max maybe 1/3rd of an inch. I do this a few days in a row carefully to make sure I get my whole head and that I don't take off too much at a time, then as a final touch I do the sections again with my thinning shears, because I hate 'choppy' haircuts.
It's a kind of complicated process, once my hair is longer it will be easier for me to trim myself. I maybe trim it this way once every 3 or 4 months, but try to go as long as possible.