It took me way too long to figure this out. Until a week ago, I would blast the filter with compressed air. What seemed like a dozen cycles and an hour later, the job would be done. One late afternoon, staring at yet another clogged filter, here's what I did:

1) I tracked down the adapter plate for the old style cylindrical filter, it looks like this:

You'll need two of these if your vac has a cylindrical filter. Alternatively, you could look for a press-on filter for your vac. This type:

2) Next, a deep-dive in to the spare parts bin to find a 8mm (5/16 inch) steel rod that was about 1.5 times the height of the filter.

3) Put the adapter plate on the open end, then skewered that assembly with the rod. Here it is, ready to go:

4) Set the vac to blow, with the concentrater attachment at the end of the hose.

5) Hit the power, and hang on! The filter rapidly spun up past 1,000 rpm, or so it seemed, and a cloud of dust, dirt and dander went flying.

6) I moved the blower wand up and down the height of the filter a few times, then changed rotation direction just for fun. A full minute later, there is nothing visible blowing off the filter and it looked to have its' original blue color.

Disclaimers:

You might not want to try this in an urban area, it may turn the neighbors hostile. However, my next HackADay project might be filter spin-chamber with an exhaust debris capture bag.

The photos above were shot with a spare filter that had been cleaned with the method described.

I guess I'll have to post a short video of me airing my dirty filter some day.