How do I not get lost underground?

I have found a rather large series of underground caverns in Minecraft, and have come close to becoming fatally lost several times. What are some best practices in making sure I get back to familiar territory? I have the compass but since it points to my original respawn point it's not terribly useful. What tactics are employed to make sure you can explore deep caverns and still make it home in one piece?

There are a number of spelunking techniques that lead to successful navigation and tracking while wandering underground. As with anything in minecraft, you'll want the right tools for the job:

tools

1. bow+arrows (sword optional)
2. food
3. torches/raw wood
4. a bunch of stone shovels/pickaxes
5. iron/diamond pickaxe for rare ores
6. steadfast resolve in the face of impending lava, and/or water bucket

I put the bow and arrows first as they're really the most important for surviving caves in minecraft, killing enemies at a distance will significantly reduce the amount of damage you take, which will allow you to go further, although food will help in the event that you get hurt.

Torches are key, you'll need lots. I usually pack 64 and make more from wood when I find coal veins.

stone shovels and pickaxes are just for regular old digging, and you'll need to do a bunch of that to get through the steeper jumps

As far as directions go there are a number of simple tricks to being able to find your way around.

lost & found

1. Right/Left-wall rule – Follow the right walls while traveling into a cave, and you'll be able to do a 180 and follow the left walls to find your way out. This is a tedious rule to follow, but is one of the simplest rules for new adventurers to adhere to. As an extension of the rule, place your torches on the same side, and you'll have a better idea of whether you're going into or out of the caves.
2. Signs – Signs used to take up a bunch of inventory space because they didn't stack. But they are stackable now, and they make for great posts for spots that are particularly confusing, especially when you find yourself at a 7-way junction that goes every direction but up.
3. Arrows – Not the arrows for your bow, simply arrows made out of blocks, use dirt/gravel/cobblestone to make → in the walls or ground. Have fun and make a 3D floating arrow in the middle of the path, it'll be much more visible than a sign, and can be a useful way of getting rid of some of the materials that you've no doubt collected on your journey.
4. Torch Patterns – use torches to make patterns to give you clues as to which way to go. Simple designs like ..: can be used to point the way in or out (make up your mind before you go in so that you're not confusing yourself with inconsistent signage.
5. Drilling – If you've got a particularly deep system of caves that double-back and overlap, "drill" some 1x1 ladder holes or 2x2 staircases in any location. They're convenient fast-access points to the surface or other tunnels. As long as you can find your way back to the surface, you're bound to be able to get back to your home.
6. Clean-up – If you've got a lot of time to burn, and you really want to be able to get home in one piece, dig out the tunnels so that they follow a nice pattern with decent lights and stairs.

If you place cobblestone on the floor and look at the top, you should be able to see a L-shape (only with the original texture pack.) If the L is the correct orientation you are facing North

If you saw this texture then you would be facing North. This can help you keep orientation if you know which way you are suposed to be heading.

If you're not averse to changing your texture pack, this can be a great solution. Alternate texture packs will change the look of your paintings as well as the rest of your textures. For example, the PieHole texture pack transforms some of your 2x1 signs into these directional arrows:

The texture pack also comes with some good paintings for marking dangerous areas:

Pros:

• Unlike signs, paintings can stack in your inventory, meaning you can carry as many of them as you like without filling up your entire inventory.
• These bright, & colourful 'paintings' are much easier to identify than standard signs, and don't require you to read them.

Cons:

• You may not want to change your texture pack.
• Paintings are randomly selected when placed, so it might take you a few tries to get the one you want. A trick to speed this up is to temporarily build around the spot you want to place your painting in; this will limit the selection of paintings that can randomly appear to the ones that can fit inside the resultant shape.

EDIT:

As Kevin Y pointed out to me, if you really don't want to change your entire texture pack, you can simply edit the paintings in the default texture pack. This will retain all of the standard textures, while letting you switch to 'custom' paintings.

If you're unsure how to edit your textures, you can read about it here.

There are some good suggestions, but a couple useful tricks are missing so I'll add them. The most certain way of knowing where you are (particularly if accuracy becomes important) is to use the coordinate system. Hit F3 and you get a HUD (Heads Up Display) which displays coordinates. Note down coordinates for places you know, and learn which direction they increase/decrease in, and no matter where you go you can find your way back there. It's worth noting that for some reason the Y coordinate is up/down instead of Z.

Another tip for exploring large caves is to set up totem poles at entrances to branches you've finished exploring. Place a stack of blocks 2 or 3 high with a torch on the front. Effectively blocking off places you're done with drastically reduces wandering/lost time.

The technique I usually use when exploring a large tunnel network is:

• Always place torches on the left side of the tunnel when moving forward. When I'm back-tracking I keep the line of torches to my right.

• Keep a full stack of dirt blocks in case I'm hopelessly lost and just need to tunnel up to the surface.

• If I enter a large cavern with multiple exits, I'll mark the tunnel I came in from with a few blocks of dirt (assuming the cavern is mostly stone).