Going back to what tricky had said some time ago:
You can try starting your backswing with a pronated arm with the racquet pointing slightly forwards, like Federer does. It will be much easier to keep it straight at that point. You will have to flip your arm quickly into a supinated position just before the forward swing to point your racquet backwards, and maybe it will be easier to keep your arm straight with this sequence.
That said, I would recommend doing what comes naturally. I don't have either the strength or enough fast twitch muscles in my arm to do all the things that ATP pros do so quickly, so in my takeback, my arm is not as pronated and my racquet is partially pointing backwards already. In other words, if I try to flip as quickly as Federer or Nadal, I will goof up on fast balls more often than is good for me! It may work for you/others differently. Please don't blame me if you forget how to hit your forehand as a result!