EDIT: I tried out being Rapunzel, and it was every bit as wonderful as I had always hoped. See the instructable here: http://www.instructables.com/id/Surprisingly-Easy-Rapunzel-Costume/
So when my friend wanted to do a costume with me which would require my hair to be Rapunzel length, I started researching hair extensions.
Hair extensions are crazy expensive. You can spend hundreds of dollars on that nonsense. So that was out, and I pondered for a time. But then I happened to stop into a wig shop, and they had these $3 bundles of long, braided hair. For three dollars a bunch, I had to give it a try- and I'm pretty happy with how it turned out!
So here you go. How to get ridiculously long hair, for about $6.
Step 1: Materials
The darker one is more my color; I got the other one to blend and add highlights, as I'll show you in the next step.
It was also integral to the braiding process, and my friend and I worked it in in such a way that the pure blonde didn't show much.
You will also need a helper. Danger is my middle name did the actual braiding in these pictures.
Hair elastics are necessary as well.
Step 2: Blending colors
Note: be very careful when handling your synthetic hair. You may be tempted to unbraid a pack of hair and wear it around your neck like a scarf, but this will result in hopeless tangling! Touch it and leave it unbraided only when necessary.
Unbraid both colors of hair. Drape the color-matching one over your hand.
Take a chunk of the "highlight" hair and drape it on top of the other hair.
Finger-comb gently until the two colors are mixed.
You should end this step with a nice, color-blended bunch of hair and a highlight-only bunch of hair.
Step 3: Section fake hair
Separate it into three separate, equally sized strands. Drape them over something (e.g. the back of a chair), individually.
Step 5: Add false hair to each piece and begin to braid
Try to work the real hair around the fake hair, so the fake hair is mostly obscured by real hair.
Begin, carefully, to braid. The fake hair should stay in just by virtue of being braided.
Step 6: Add more hair
Take the nice, matching hair and put a hairband around its center, so the (unbraided) hair is doubled over.
Tie it into a strand of hair from the braid that is attached to your head.
Part the doubled-over (nice, matching) hair into three equal pieces.
As you continue braiding, add these pieces to your original three strands.
This should cause the newly added hair to make up for the ending of your real hair, so the braid doesn't suddenly become very thin.
Step 7: Finish braid
Theoretically, you could keep adding more fake hair every time you get to the end of a type of hair. I haven't tried that (yet), though.
When you get to the end, tie it off. You're done!