"Professional" stool collection kits include a test tube with formalin. I just ordered some formalin so that the next time I pass a worm, I can preserve it and take it to the doctor:
I expelled a few worms just now, after taking 4 tsp of Pin-X last night (hmm, Pyrantel does work). I have no idea whether these are just pinworms or Strongyloides, and if I had a small container of formalin I wouldn't have to wonder.
Next time, I will be ready.