I keep a spray bottle full of RO/DI in the area where I do my tests. I sray it inside the test viles out as a final rinse. The spray bottle is also really good for cleaning water spots off your tank too.

You can get a spray bottle at Wal-Mart for a couple of bucks. I also have one with vinegar for cleaning pumps. If you have more then one, its essential that you label them clearly.