This is my one block elevator shaft.

You need the following:

A few buckets of water
4 slabs
Signs 
Torch (optional)
Glass or glass pane (optional)
Glowstone

First of all dig a hole. Under it is my base and my mine.

It is up to you whether you do it upwards or downwards. Although I prefer downwards to prevent suffocation from falling sand or gravel. Next put signs (leave some spaces to have some room for the water.

Then add the water between the spaces.

The water should make you float and the signs will provide enough space for you to get air. The elevator should look like this underground. As you can see it is elevated.

Just putsome glowstone or torches like this so that you have lighting.

-> Just press up (or jump) and you will automatically float. The signs will refill your breath.

Submitted by: benard88