How do I safely redirect lava?

With 1.1 the previous answers are no longer correct. Since you are digging down through the lava, redirect some water into your mineshaft. As you dig into the lava, the water will hit it and form obsidian walls to hold the lava back. You will need a nightmare pickaxe or better to dig out the obsidian that's in your way.

Another alternative is to use an obsidian skin potion (and maybe a gills potion) to safely swim to the bottom of the lava and dig a drain for it.

You can place blocks two spaces away from you. Take advantage of this to fill the lava over with stone or something similar as you go; when you mine the filler blocks, they will turn to air, and you can force a wall between your tunnel and the lava.