How To: Avalanche Mechanics

Avalanche mechanics is the term used to describe the physics of sand and gravel. Both can be supported by stable objects such as dirt, stone etc, but objects like torches and ladders work as well. Normally it is used as a gate, or a part of a trap, due to the fact it "closes" that area after the stable object is destroyed. Below is a basic example, and a basic setup.
Before

Note the dirt, used to quickly block off access.

Before

After

If you would like to deviate from an avalanche being used to only block off access to an area, put some kind of danger in the middle of the gate. So either lava, landmines or cacti. As soon as the mob nears, block off the entrance and they'll fall prey to your trap.

There is one thing that may be difficult, the range factor. You have to be within several blocks of the trap for it to be effective, which may be a huge problem when dealing with skeletons. With the below variation, though, you can do it from range.

So torches have two unique properties. 1.) They can hold up sand and gravel. And 2.) They break when touched by water. So simply have a torch holding up several blocks of sand, an aisle where the water can be poured(note, it has to go from a higher elevation to a lower one to achieve max range) then an outlet so that the torch will break and the way close.
Gate: Open

Waterway: Filled and beginning to flow

Gate: Closed and secure

So, use the above methods to fortify your base and protect your pixelated goodness.