Whenever you need to keep your writing from prying eyes, lemon juice invisible ink will do the job quickly.
Getting information and Communicating are the two most important skills of any spy. If you communicate by means of paper, you basically have two choices: coding and cloaking.
Coding has one problem. Everyone who comes across an encoded message, knows that this is a secret message. And that you are (most likely) a spy. Your cover blown. Bad news!
Another possibility is to cloak your message. In other words, make it invisible to the naked eye. You can make invisible ink, and write with that. Even better: encode your message first, THEN write it in invisible ink. When it comes to your safety, there's no such thing as doing too much.
If you have little time, you can use a lemon to make invisible ink. The trick with lemon juice is that it IS invisible ink. You only have to press the juice out of a lemon, dip a crown pen or pointed feather in it, and write your secret message. Because it's colourless, it can't be seen after drying.
Only when you heat the paper, f.i. above a candle (make sure you don't burn the paper!) or with an iron, the lemon juice ink will discolor brown, and you'll be able to read it again.
Tip 1: Don't write your message on an empty page. An empty page is strange... why would you want to send an empty page to someone? When people start asking such questions, they may investigate further. So make sure you write a message on it. Something harmless, that anybody may read. Another idea is to write a message with loads of clues that lead nowhere, throwing your enemy off track.
Tip 2: Lemon juice invisible ink, while quick and handy, isn't completely invisible. It's a good idea to write on paper that's not white-white. Recycle paper (as used by newspapers) is ideal for this.