Put a teaspoon of washing-up liquid diluted with three teaspoons of water into the clean glass. Swish the salty water around your mouth vigorously for 30 seconds or so then spit it into the diluted washing-up liquid. Stir this firmly for a few minutes, then very gently pour a couple of teaspoons of ice-cold strong [> 50% by volume] alcohol down the side of the glass. ... [Y]ou must have a clearly demarcated water/alcohol boundary.
The result? "Wait a few minutes and you'll see spindly white, thread-like clumps starting to form in the alcohol. This is your DNA." This delightful method comes from an even more delightful book called "How to Fossilise Your Hamster." It's a must-read.
Warning: non-medic friends might think you a little strange. But what do they know?