Brenda's method is spot-on.
Two things that I had to learn - or rather, that my cats taught me -
1. You don't have to use overwhelming force & violence (shock &awe) to get this done. It can be done smoothly and professionally, and the cat prefers that you do it right. Gentle pressure on the side of the cat's jaw will open it.
2. anything placed at the back of a cat's tongue MUST be swallowed. The way a cat's mouth and tongue are made, the raspy grain points down the throat. If the pill isn't swallowed, you didn't get it back far enough.
That said, once in a while I will fail to get the pill placed properly on the first try, but the second try works.
Here's a bit of information that I think EVERY cat owner should know: If your cat is achey - from a fight or arthritis or a fever or old age, he will BENEFIT GREATLY from one half of a baby aspirin. Best of all, the pain relief and anti-inflamatory action last for about 3 days from a single half-tablet dose. I got a small bottle of enteric (coated) baby aspirin and a pill splitter that I keep in "the cat drawer." I don't medicate him just for the heck of it, but when he's feeling crappy - like after he's received three vaccinations at once from the vet - a half baby aspirin helps him feel better.
My vet also told me that cats enjoy the herb Valerian (from which valium was discovered) almost as much as catnip. I have not confirmed that yet.