BIF is one of the most critical aspects of bird photography.
I have seen some people in a hurry of taking BIF and don't take enough care and touch a lot the focus ring of the lens, you know what that lead...
If we exclude all gear issues, you need a lot of practice. Practice following the bird and shoot during the panning and DON'T STOP panning after the shots. Follow-through is critical here.
Now the gear
Lens
1. AF tuning (portraits are closer than BIF, so try to AF tune at the expected distance (I do mine at around 50M))
2. With the 400 f2.8 use at least f5.6 until you reach good in focus picture, than start to go up (in speed) as you get better or respective to wich effect you are looking for
3. Upper than 1/500 VR off. Ok, it should always be off, because I recommend to shoot at around 1/1000 (start at day light to exclude other issues like slower speeds and open fs)
4. Don't lock the lens foot, but use it snug
5. Don't touch the lens, use only one hand in the camera body
Camera
1. AF-button
2. AF-C
3. Focus priority (to begin and learn, later you should have shooter priority)
4. Focus group (dinamic later on)
5. Focus on hold slow (to fight busy backgrounds)
6. High speed shots (use short burst 1~2 seconds)
BIF approach
1. Spot the bird
2. Start to follow it
3. Understnad where it should go
4. Start against blue sky (exposure ~+1)
5. Shoot and enjoy