I find it best to warn them before I pick them up. Even if they are looking at me, I like to stroke their back a few times before I go to lift them, so they know whats about to happen. I'm sure it's completely unnecessary, but I feel it's polite if anything else. Emrys has had a few times where he was a bit cranky and looked like he might want to strike, and I just left him alone for a day or two and he was fine to be handled again.