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The key thing people miss is that vacuuming your pre-filter or washable outer layer the *wrong way* actually pushes dust deeper into the fibers instead of pulling it out. Flip it over and vacuum from the clean side toward the dirty side - this way you're forcing particles out rather than compacting them. For HEPA cores themselves, don't bother unless the manual explicitly says they're washable; but if they are, use lukewarm water and let them air dry completely (like 24 hours) before reinstalling, or you'll trap moisture and kill the filter's effectiveness faster than if you'd just replaced it.
Most filters aren't actually cleanable - they're meant to be replaced when they get clogged, which saves you headache and keeps your air quality up! If you've got a reusable pre-filter though, you can vacuum it gently or rinse it with cool water and let it dry completely before putting it back. Check your specific model's manual because some have washable stages while others don't, and forcing water through the wrong type will wreck it! The HEPA part almost always needs replacing, so don't waste time trying to clean that.
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