Your left arm will always bend a little but the idea is to try and keep it relative straight and relaxed. I think one of the major causes of this is linked to the right arm collapsing too early on the back swing. Try and keep both arms relatively straight to way past hip level, this will also help you to create swing width.

Here's a tip I use: Get a plastic 1.5 or 2 litre pop (soda) bottle, wash it out and cut off the top and the bottom. Slide the bottle up your left arm like a sleeve. This should prevent you from bending it too far. Hit a few balls with the bottle on then try to recreate the feeling wothout the bottle.

Paul