Yep, shooting a moving object while getting the 'blur' to indicate speed is a tricky one.
I believe your camera has 3 focus points? If you are shooting something moving left to right, I would set either the left or the middle focus point (don't let the camera decide which point to use), set the AF to continuous and the shooting to continuous too (you'll need a decent memory card, or you won't get many shots before it starts slowing down).
You need to start panning before your subject gets to the point you want to take a pic and follow through with a smooth motion. Even when the subject has moved past, keep panning and you'll get a smoother result. Keep the AF point on the subject.
If the camera's AF can't keep up or gets confused then try putting it back to single AF & pre-focussing on something, then pan and start shooting as the subject gets into focus.
It's not easy! Well, I don't find it easy, anyway!!