You are going to need to see some hidden files for this.
1. So start by opening Terminal.app and pasting in the following (don’t worry, we’ll undo this change when we are done)…defaults write com.apple.Finder AppleShowAllFiles YES
2. killall Finder
3. Download the installer as normal from the app store.
4. After download you will have the ‘Install OS X 10.9′ app in your /Applications folder. Right-click it and select ‘Show Package Contents’.
5. Navigate to the Contents/SharedSupport. There you will see the InstallESD.dmg file. Unfortunately, you can’t just grab it and restore it. Instead double-click it to mount it.
6. You should now see the normally hidden BaseSystem.dmg file in the mounted volume.
7. Open Disk Utility and restore the source BaseSystem.dmg to the destination of your USB drive.
8. Now open up the restored USB volume (default name is OS X Base System) and navigate to System/Installation. In there you will see an alias for Packages. Delete this alias.
9. Copy the full Packages folder from the root of the mounted InstallESD volume (the same place you got the BaseSystem.dmg file) to the USB volumes System/Installation folder. Basically, replace the Packages alias you just deleted with the real Packages folder from the original InstallESD volume.
The USB drive should now be bootable. So simply restart, hold down the Option key (Alt) when booting with the USB drive in your machine, and you should get the option to boot from the USB drive.
Remember, to rehide the normall hidden files just repeat step 1 but change YES to NO.