** Edit from the admin **
I'm leaving this in because it does not violate any forum rules specifically, though I would like to remind whoever reads it to look up information in a original and verifiable source (i.e., a book, encyclopedia, or supervision from a professional) rather than take for granted what is on the web, as this information is not verifiable.
** /Edit **
Obviously, don't make a lot of it if you do synthesize any. Store in a brown glass jar (actually a tight-lidded HDPE container stored in the dark may be better..in the event of an accidental explosion, no shrapnel) with an airtight lid. Store in a cool place, keep the contents wet with water...slightly inconvienent if needed for something, but greatly reduces the chance of accidental detonation/ignition. Also, keep it away from contact with metals. Metal picrates (notoriously lead picrate) are much more unstable explosives than picric acid itself. Stored properly, picric acid is just about as safe as any other compound you might have around.
The storage out of light in a cool place with as little air contact as possible is a good general storage procedure for just about anything.