Just as your car needs its oil changed, your fountain pen needs internal cleaning. A once a month cleaning will often save you a trip to the nib specialist. After flushing the ink out of the pen with water, we recommend flushing with one part household ammonia* and two parts water. Fill and empty the pen three or four times to dissolve the solids accumulation. Then flush the ammonia solution out of the pen with three or four more flushes with tap water. This is especially important if you change brands or even colors of ink, or use a cartridge exclusively (some inks react with each other, creating a viscous substance).
Even if you just flush several times with tap water on an irregular basis, you should be able to avoid most clogging problems
Many piston-filler pens, such as Pelikan and OMAS brand pens, will begin to twist hard if ink is allowed to cake inside the barrel. In an extreme case this can compromise the gasket or even break the twist mechanism. If you can see accumulated dry ink in the transparent part of a Pelikan pen, you can see how this problem occurs and take remedial action yourself.