This is a tutorial on making an automatic incinerator.
- First, build a repeater (5-sections or rapid)
- Then, make a lava curtain (see photo)
- connect a dispenser to your repeater and put the items to be destroyed in said dispenser.
A fence around the lava might also be a good idea.
Enjoy your automatic incinerator!
Reapeater blueprints: http://www.minecraftwiki.net/images/c/cf/StandardLogicGates.png