Usually, if I'm not in my studio, I just blow on the thing really hard from both sides. This isn't really good enough but it makes me feel better. I've washed them, blown them with compressed air, and gently cleaned them with a needle. The last mentioned requires a very delicate touch, but that is the way I get them clean when I make them and I've had a lot of practice. As I've grown older, though, everything about making and maintaining pinholes has become much more difficult as my eyes become less reliable and touch loses some of its steadiness.

I have to wonder about using some sort of sticky stuff to collect transient particles, a kneaded eraser might work (the kind that we use in charcoal drawing).

Maybe some sort of electrostatic device could work; I have one of those gizmos that labs used to clean negatives, but I haven't yet tried running a pinhole through it.

It's sort of impractical to carry an air compressor around all the time. Another idea that seems promising would be to carry a small graduated tube to blow through, concentrating a stream of air. Washing in water is practical, but the hole needs to be dry; a drop of water becomes a lens.

Regardless of what you do to clean a pinhole, it is important to be able to observe its condition. I use a 50x projection microscope to see what's going on if I have it available. Out there when working, a pocket microscope can be helpful if I remember to take it with me. A loupe or magnifying glass doesn't magnify enough.

Or, of course, you can just live with what you get. I think the most important thing is to start out with a clean pinhole and try whatever you think might work when you are using it. If the hole has ragged metal left in it when it's made, it will affect both exposure and image quality.

Now, I must confess that I really like a "sharp" pinhole image, to the point where my pinholes have been criticized as "too damn sharp". The quality possible in a pinhole image can be really amazing. While they don't have that kind of sharpness that lenses can produce, there is a potential for sharpness embedded in softness, if you can visualize what I mean. It's hard to describe.