How to fly in minecraft

It's fairly easy.

There are a number of client-side flymods out there, the most popular being Zombe's Modpack. Just follow the included installation instructions and press F to toggle flying.

As of Beta 1.5, flying is (per default) disabled on vanilla servers. You can reenable it by setting allow-flight=true in server.properties. If you want to allow only certain groups to be able to fly on your server, I highly recommend getting Bukkit plus an anti-cheat plugin like NoCheat.

Creative mode in 1.8 has inbuilt support for flying. It is available by double tapping space.

Reedit: Using these instructions, you can switch a survival map to creative:

1. Download NBTedit from here
2. Unzip the ZIP file
3. Run NBTedit
4. Click File -> Open
5. Navigate to your save folder
6. Open level.dat
7. Switch gamemode to 1
8. Save and exit.

In multiplayer, ops can type /gamemode [playername] [1 for creative, 0 for survival]

Edit: This is wrong! There is a /fly command you can use! same controls

If you use "Single Player Commands" you can:

1. press "T" to open the Console
2. Use the command "/noclip" to allow you to fly around, however you can also "Fly" through blocks,.

Controls for /noclip flying:

WSAD: Move like normal
Shift: Go down
SPACE: Go up.

This isn't really "flying" in the sense that the Original-Poster meant but you are flying!

Edit: This is for Single-Player, not Multiplayer. Hopefully this will help someone who wants to fly in single-player but I don't think it works on multiplayer.