Like with so many things in guitar, you need to start slow, and then go faster and faster and faster. When you get up to speed everything should be happening naturally. Another thing I have noticed when people pick up and strum a guitar is that they strum it really hard. There's no need to apply much pressure, with you right hand, on the strings as you strum slowly. Everything is about accuracy rather than about speed because no matter how hard and fast you play, if you're not accurate it sounds bad.