How can I decline a call in iPhone when the screen was locked?

Push the power button on the top of the phone twice. The first time, the call is silenced. The 2nd time the call is rejected/directed to voice mail (if set up).

When the phone is locked (the screen is off)

On iOS 6, it show slide to answer and a little phone, that, when you slide it up, shows more options, such as accept / decline and more options.

On iOS 5, it only shows slide to answer. Like Mark said, tap the power button once to silence the call, twice to reject it.

When the phone is in use (screen on, unlocked)

On All versions since the beginning of time, it will show Accept / Deny buttons.