The problem is that live animals swim around and go out of the field. They also go up and down and go out of focus. That means you need to slow them down.
The three main ways of slowing animals down are:
* to put barriers in the way, so the animal can still move as fast, but not as far;
* putting the animal in a more viscous (sticky) solution which usually kills them in the end; or
* kill them outright.
The most common barriers are bits and pieces of cotton wool or ground-up face tissues. This is not very effective with anything smaller than a mosquito wriggler, but it's better than nothing.
Live specimens can be mounted successfully in ®Gurr's Water Mounting Medium, which slows them down (and kills them). I have been using the same bottle of this product for almost 40 years, and it seems to be hard to buy nowadays, though it is still mentioned by professional scientists.
A solution of 10 g of methyl cellulose in 90 mL water forms a syrup that will slow most animals down for microscopic examination, while allowing observation of movements of the gut, breathing tubes, and so on. You can buy methyl cellulose at hardware shops, though you may also get it at health food shops, where you will probably pay a lot more for it. Be careful not to get it in your eyes or on your skin.
I haven't tried this, but I'm told you can also add 2-3 grams of gelatin to 100 mL of cold water and heat this while stirring. Cool the gelatin solution back to room temperature and add one drop of pond water to one drop of gelatin solution.
If you mount the animals in 70% alcohol, this will kill them, but a 1% solution of magnesium sulfate (often sold as "Epsom salts") will just anaesthetise them. Note that 1% here means 1% by weight or one gram in 100 mL of water. Put a drop of this on the slide and then use a camel hair brush to add the animal.
Just a reminder for those coming in late: the material I am posting here is made up of out-takes from an upcoming book Australian Backyard Naturalist, due out in May 2012. This is the stuff that won't be there.
Next time, I will look at catching nematode worms.