What Sadie said - I try to clip in as I'm going into the downstroke on that side. That way I have the motion of going down/forward already so it's just a matter of clipping the foot in. I start with the clipped in froot in the up or 12:00 position so I have the full rotation to get the bike going and/or get the other foot clipped in. Another thing you might look at - are you trying to clip in while standing or seated during take-off? I used to try and clip in while standing and I kept sliding off the pedal. Now I sit down on take off, clip in and then stand. Hope this helps!
Life should NOT be a journey to the grave with the intention of arriving safely in an attractive and well preserved body, but rather to skid in sideways, champagne in one hand, strawberries in the other, body thoroughly used up, totally worn out and screaming: "Yeah Baby! What a Ride!"