I far too frequently let ink dry inside one of my treasured pens. When simply flushing with hot water won't clear them,I use this cleaning solution.
Simply draw some solution into the pen as you normally would ink. Shake it around and then let sit for 5-10 minutes. Flush the pen out several times with warm water, and you are back to an evenly flowing pen. A wonderful practice is to rush warm water through the pen between fill ups. I often forget.
J. Herbin Fountain Pen Cleaning Solution 15Ml via Amazon
Source: boingboing.net