How do I make a thin layer of lava for mob farming?

In order to create a lava trap that damages monsters but does not destroy items, you want to keep a ratio of 1 bucket of lava to six blocks of trench.

This will allow the lava to burn monsters (and you if you don't make an obsidian skin potion!), but will not be deep enough to destroy the drops -- even Coins are safe.

I didn't have a bucket of lava handy, but this is what your moat should look like - though I had to use water, which is significantly less lethal.

The last thing to note, is that your liquids may get, for lack of a better word, "stuck" on the edge of the trench. In that case, all you need to do is exit / reload the world, which will settle the liquids, and appropriately depth your trench.