Whenever I'm available for commissions, the first thing I do is to contact my regular clients and ask them if they want something. Only if none of them are available or interested is that I open commissions publicly.
Once I post about it (usually on Twitter), I'll wait until a couple of people show interest, and then I decide with who I'll work with depending on how much I like their project idea.
This system is pretty comfortable for me because I'm already familiar with my regulars and we've already learned how to communicate with each other. The process of learning how to approach new clients and learning their tastes every time can be overwhelming to me.
I'm terrible at predicting how long any given piece will take to finish, so reserving commissions slots "for whenever I'm free" gets kinda awkward, especially when a commission takes a long time and there are multiple people in line. There's been many times when I had reserved a slot for someone and then it turned out that they weren't available when I was free. And then they'd be available again when I was in the middle of a new project.
For all these reasons, I very much prefer to only open my commissions publicly if none of my regulars are available. If there's no indication that I'm open anywhere (not on my twitter, not on my Newgrounds, etc.), we can't discuss commissions at the moment.
I appreciate the interest in commissioning me though! And I hope we get to work together on something some day.