The cleaner you can keep hydraulic oil, the longer your hydraulic components will last. After you're sure filters and oil are being maintained properly, here are some additional ways to keep dirt from sneaking into your systems.
If you're working in lots of fine dust, banked rock, or corrosive material, protect rod surfaces with heavy-duty covers. Maintain the seals and they'll stop a lot of contamination.
Use tight-fitting drum covers. Store drums of hydraulic oil indoors if possible, and always cover drums with lids that will keep out airborne dust and moisture.
Any time hydraulic hoses are disconnected, whether they're in the storeroom or hanging from a machine, they should be capped to seal out dirt.