Make sure you take it out of it's mountings every couple of months or so and give it a good shake. If not, when the need arises you'll find all the powder is likely to be one compacted mass that the propellant cannot force out of the extinguisher...
Graham (G4FUJ)
Sold L44FOR 4/4 Giallo Fly '11 MINI Countryman Cooper D All4 '90 LR 90 SW