There is no real perfect solution. No matter what you do, someone dedicated to it will be able to steal it.
Even Twitter for iPhone/iPad/Android/mac/etc. has a secret key in there, they’ve likely just obscured it somehow.
For example, you could break it up into different files or strings, etc.
Note: Using a hex editor you can read ascii strings in a binary, which is the easiest way. By breaking it up into different pieces or using function calls to create the secret key usually works to make that process more difficult.