You can't stay positive if there's no substantiation behind your positiveness. So don't bother. It's a house of cards if you try.
Positiveness and confidence don't come that cheaply.
With that said, you'd need real substance, ie skills, proofs, understandings. Get your skills up, or know your opponent's weakness and your own strength and exploit it. If it's true, it will only take a couple good shots for you to boost your confidence or affect your belief.
Almagro wasn't as much negative as the limitation his skills hit. Ferrer ranks highter, plays 5 setters better, etc. Ferrer is a walk over for Djokovic and who knows where these guys' confidence level really are!