Ink should be kept in a cool dry place. In class, we were taught us to apply a generous dollop of petroleum jelly (like Vaseline) around the opening before replacing the lid to better seal it. Sometimes, however, the top portion will get dried out despite proper storage.

When that happens, use the ink knife to carefully scrape around the edge and fold back to the dried part to expose the useable ink underneath.
What about the storage of smaller amounts of ink that you custom mixed? On his website,
Dave Celani describes how to fill small tubes with ink. If that sounds like a lot of work, I think an effective - although not as elegant - technique is to simply scrape the ink onto a piece of wax paper, fold it and then place it into an airtight plastic bag. I recently used some of the brown ink from the baby announcements which I stored about a month ago, and it was fine.