Help! I got lost in Nether!

I believe, basing on ReallyGoodPie's answer, I do know something that would work most of the time.

Unless you're a devout traveler and wandered for days away from the spawn point (not too smart, a creeper blows up you and your bed and you're back to spawn, days away from home - not to mention all three strongholds are between 640 and 1152 blocks from the spawn) we can assume your Nether Portal was somewhere within, say, 3200 blocks radius from 0,y,0 (that's like 9 maximum zoom-out maps!). Since coordinates in Nether are an eighth of the Overworld, that means the portal will be within 400 blocks from 0,y,0 (and you can estimate the radius by estimating the distance of the overworld portal from spawn.) You can also estimate the altitude (y) - 64 if that was the surface, more for hills, less for underground.

Now locate 0,y,0 using F3 and then travel in spiral around that point, trying to explore increasingly wider circle of terrain and controlling your distance using the coordinates. That way you shouldn't miss any terrain and be able to locate familiar areas and eventually the portal - 25 chunks radius isn't something beyond hope for exploring, and most people will have their homes (and portals) within 1000 radius, that's mere 8 chunks of Nether away from Spawn. (and if you remembered rough direction of your portal from spawn, using the Cobblestone Compass trick you can head straight for your portal from the "origin")

Another option, inspired by Loren: If you have enough wood, or a chest - or are near enough from the portal (say, aforementioned 0,y,0) - or are patient enough to locate a treasure chest at a fortress - write down your coords, dump all your equipment, suicide, then go, retrieve it all - stopping long enough to take note of the portal location this time.

Everytime you go into the nether, I always usually do a couple of things, especially if it's my first time entering. Here's what they are:

• Take note of the coordinates. I usually open up notepad (or text edit on Mac) and then hit F3 while in Minecraft, and a bunch of text will be displayed like the image below. If you look where I have circled with my professional paint skills, you will see a bunch of coordinates. Take not of these in you text editor as you first step through into the nether and this will tell you exactly where your portal is. Be sure to take down the x, y, and z coordinates. IF you get lost, simply try and head back to these coordinates (simple enough). You only have to do this when using a new portal, as it will never change.

• Place cobblestone trails. Cobblestone does not naturally generate in the nether and due to the dark colors, it "stands out" quit nicely, meaning it is hard to lose your way.

If you are seriously already lost, there is no hope. You can walk around for hours upon hours only to realise (or not) you've been walking in the opposite direction of your portal. Always try and prevent this stuff before it happens.

Prevention is the best cure... or something like that

Bring enough stuff to create a second portal. You will need the following:

• Flint And Steel (to ignite the portal)
• 10 Obsidian blocks (for the frame of the portal
• Bring a compass

When you get lost, simply build a portal and go through it. Upon entering the overworld, you can use your compass to find your way back to your spawn point.