I generally never "fill" mine more than 1.5 inches. I find that loading more than this results in unwanted and uncontrollable flow. When finished, I just blow air through it to evacuate remaining cement. Allowing the unused cement to evaporate on its own will most surely result in a clog.
As for cleaning, Obtain the following:
1. 1 qt. can of laquer thinner.
2. Plastic droppers from the Nail Tech section of local beauty supply store. They have a more robust end and they're cheap.
3. 2" section of 1/8" automotive vacuum hose.
Place one end of the vacuum hose onto the end of the paint dropper to about 3/4".
Depress paint dropper to greatest extent possible - holding it.
Now, with your 3rd hand, insert glass end of the clogged touch n flow to about 3/4". (Seriously, get help for this since the glass end of a touch n flow is quite easily broken.)
Place metal end of the touch n flow into the open can of lacquer thinner, allowing it to stand upright. The paint dropper can flop over as it may, due to the flexibility of the vacuum hose.
Watch it as the vacuum overpowers the clog and the lacquer thinner flushes it out nicely!
Over several years, I bought new touch n flows as they clogged up. Since fine tuning this tecnique, I now have four and the oldest one is still as good as new.