This how-to, although not specifically applying to nether and general lava deaths, has advice that does apply to these in-game mishaps as well.
You're mining some obsidian for a portal or maybe an enderchest when suddenly you fall into lava! Something's wrong with your water bucket or you can't seem to get out... whatever it is, you're dead now, and your items are gone, including that diamond pickaxe and your fancy armor.
You feel terrible, but don't worry: it's not game over. You can recover and do even better.
Step 0: Run and check.
If you remember where you were, quickly go down the mine and check to see if anything from your inventory didn't burn. Salvage anything? Awesome! Find nothing? That's okay!
Step 1: Take a break.
This is probably the most important step. Don't try and do anything else in Minecraft because it will aggravate the situation. Log off before you start doing damage to your world out of frustration.
Get up, drink some water, chillax. Minecraft's not your obligation, after all.
When you've calmed down, log back on.
Step 2: Get your inventory back in shape.
You probably have your own inventory layout, right? "Sword in slot 1, pickaxe in slot 2, wood and cobble in the upper left corner, etc..."
Obtain what you can from your chests. Then, go chop some wood; go farm some carrots. If you've only got a few iron ingots or diamonds, only make the most essential tools out of them. The rest will come later.
One thing beneficial about dying in Minecraft is that you've cleared out all of your useless clutter and now are carrying a light and tidy inventory.
Step 3: Go mining.
Go caving or simply dig. You'll need iron/diamonds to replenish your supply of it. Mining is relaxing and makes you feel awesome. Bring water buckets!
Step 4: Celebrate!
You're inventory's clean, you've got fresh armor (that's maybe even enchanted), and you've got chests overflowing with iron and diamonds.
Go build a fortress, shape a monument, or set off some fireworks. You're thriving! Now you just need some obsidian...
Step 5: Go conquer that lava.
I guess there are other ways to get your obsidian. Make an obsidian machine, ask your friends to do it... Nah! For the sake of personal achievement, let's go do it ourselves.
Bring
two buckets of water. Not one, but two. I'll explain why. Underground in a cave, dig one block next to a lava lake that has obsidian on top (as in, the top layer of lava has been cooled by your lava bucket). Pour 1 bucket of lava into that block. Sit in the water and mine the adjacent block of obsidian. This way, the water will cool the lava underneath that obsidian block when you mine it. Mine the next adjacent blocks, and so forth.
In the event that your water "tether" stops short and you accidentally fall into lava, use your second bucket to cool the lava as well as your character. "Not today!" you say to the lava.
Thanks for reading!
Have you ever fallen into lava while mining? I know I have, many, many times. Share your disastrous experience in the comments!