Hans is right, plenty of practice(and lots of dissapointment!) is the only way you will get panning right with any lens.
The good news is that with digital, you dont have the expense of wading through loads of poor prints, just watch them on the PC and analise your mistakes.
On the 100- 400 Canon lens, it may appear relatively expensive but with the image stabaliser seems to do a relly good job - I wish I bought that instead of the the 70-200!
http://www.jonbunston.co.uk