Minecraft wiki entry for Tips_and_tricks, associated text: Place a water source at the edge, or dig out the edge of a nearby pool to let water flow in. This will make you a waterfall that you can swim up and down.
Check for lava at the bottom first -- if your water flows onto lava, it will quench the lava as far as the water flows -- about 7 blocks. If a lava lake reaches further than that, the water can wash you straight to your doom! You might want to move the waterfall around a bit, placing your water bucket in different places to quench a wider area. Then you can leave the waterfall in the middle of the safe area, where it can't reach the lava anymore.
You can place sand or gravel at the edge, letting the blocks fall down to make a stack, which you can step onto and then dig your way down. To get back up, just reverse this with classic pillar jumping, or you can make a more permanent exit by placing ladders or building a staircase.
Slow but steady: You can dig a staircase down the side of the ravine, being cautious of overhangs and monsters.
A more advanced technique is to harvest vines with shears, place some at the edge, and use bonemeal or time to let them grow into a ladder.