Something even better than post-it notes: "Magic whiteboard paper".
Basically, it's a type of thin white plastic paper with just enough static electricity that it sticks gently to any flat surface, without glue or mess:
- Since it's static electricity not glue, you can move it around as many times as you like.
- Since it's basically paper, you can buy a roll or a stack of sheets and cut out or fold to whatever sizes and shapes you need.
- Since it's white plastic, you can write on it with erasable whiteboard markers and erase or change what you write as much as needed.
- Since it comes in a variety of sizes, you can make A4 size, table size or even wall size wireframes, depending on the size of the group you're working with.
- Since the static electricity is on both sides, apparently you can stick paper on top of it, so you could use it alongside printable wireframe elements for things that can't vary.
The only downside is, it's quite fiddly - for example, it loves to fold over then stick to itself. There's a risk of getting distracted by the stuff. But if you can tame it, it's great.
There are quite a few companies that make the stuff now, but this one seems to be the original. It's more expensive than post-its, but it's reusable, so it's not bad value for money.
I can't find any pictures of it being used for wireframes, so instead here's it being used on a plane and on a shed (from the official Facebook page):