The basic idea is to avoid splinters and bits breaking off 
  the edges by gently tapping the bat with a mallet. Really 
  need advice on knocking? Then read on. Otherwise skip the 
  following excellent advise. 

Cameron Fraser: 
  When you buy a bat, it is only lightly pressed at the 
  factory and the fibres are still really soft - if you 
  press your fingernail into the surface you'll see what 
  I mean. 'Knocking in' is the process of compressing and 
  binding these lose fibres together to allow the bat to 
  withstand the constant impact from the ball. You're 
  preparing the bat to be HIT.Your aim is to 'bash' the 
  surface of the blade - not the back or the bottom of 
  the bat (that can be disastrous!) You can use an OLD, 
  GOOD QUALITY ball in your hand or put it in a sock or 
  try different types of 'knocking in' mallets - my 
  preference is for the solid wood version. Any good shop 
  or mail order company does them for around a fiver - 
  buy one, you'll need to make use of it many times. 

  Start 'knocking in' by gently working on the edges and 
  gradually knocking them into a rounded, compressed shape 
  once you get started you'll soon see the change in 
  texture. Then keep working on the edges and the area 
  around the toe of the bat - not the bottom of the bat - 
  and think of trying to use glancing strokes that resemble 
  you edging a ball to gully, then 3rd slip, 2nd slip, etc. 
  while all the time gradually increasing the strength of 
  the impact.Don't forget to give the middle a good going 
  over but the priority should be the outside inch or so 
  of the bat round both edges and the toe. 

  How long do you have to go through this mind numbingly 
  boring routine? Slazenger recommend 6 hours - now that 
  is a long, long time. If you try to do it in good 5 
  minute blocks it becomes more manageable. Essentially 
  you don't want to think about using the bat until it's 
  had at least 2 hours(24 x 5 minute sessions) but ideally 
  if you can manage double that then all the better. If you 
  have the time and space and tolerant neighbours then the 
  process can be done in a week or so - most of us need a bit 
  longer! But there's no point going to the other extreme - 
  buying a bat one season and not using it until the next. 
  Anyway, such self-denial would be way beyond the likes 
  of me or most cricketers I know! So use the bat but be 
  sensible. 

  After the initial 2-4 hours 'knocking in,' try using it 
  for hitting short catches and then in the nets against 
  OLD, GOOD QUALITY balls and only against the spinners/ 
  slow mediums first. A new or cheap ball can do a lot of 
  damage to an under-prepared bat and digging out a fast 
  yorker in the nets can spell doom and destruction for 
  even the best prepared bat! So try and middle the ball 
  and play the bat in - resist the wild slogging for once! 
  
  Then do some more 'knocking in' and then some more and 
  then some more and then some more... 

  Remember, SOME bats need a LITTLE linseed oil - but no 
  more than a couple of teaspoonfuls per season! However, 
  ALL BATS NEED KNOCKING IN. Gunn & Moore now provide this 
  service for around 10 pounds in their GM NOW range. For 
  most people that's 10 lbs well spent. But don't forget you'll 
  still need to keep 'knocking in' throughout the life of 
  your bat, knocking out indentations, evening out dead 
  spots in the bat, strengthening area around glued repairs 
  etc. A cricket dealer I know recommends that the day you 
  stop 'knocking in' is the day you throw the bat in the 
  bin.