Necro thread, we feed on the dead. <grin>
It's worth considering what "keeping track" means. At one level, it's just making sure you don't turn his brown eyes blue, and others have noted tools for that. Those are things that are fairly straightforward, both to track and to check.
More slippery are things like voice. This one bedevils me, sometimes. I like to make my main cast (primary and secondary characters) each speak in a way that makes them identifiable, if not in short sentences at least over the course of a monologue. But, my characters tend to grow (oh all right, change) over the course of the novel, and sometimes that leads them to change how they speak. Usually it's small things. This one never uses contractions. That one doesn't swear. Another sounds education.
But only by chapter 12, right? There's an evolution to how they talk. Sometimes I can set a tone and keep it from first to last, but other times ... not. This extends to how they dress, maybe how they fight, likes and dislikes, even down to the words I use to describe them in narration. Combing through the novel looking for character consistency can be wretchedly tedious. I often miss stuff.
It would be nice if there were software where I could tell it to pull out all references to Character X, including all their dialog, and let me peruse that separate from the novel. Or, at least, that'd be good for an editor!