Removing knots is not fun.

I've used tiny crochet hooks or darning needles to help loosen the knot and slide each hair out of the knot as gently as possible.  If the hair is severly kinked - I then trim it (to prevent it from starting another new knot!).

If you get to the point where you cannot slide any more hair out of the knot, try oiling or conditioning the knot and then sliding more hair out.  As a last resort, cut the knot out.

Prevention .... depends on how you think the knot started in the first place. 

Was it due to rubbing on clothing?  (I get these at the back of my neck in the winter).  If so, consider wearing a smooth scarf over your clothing to minimize the friction.

Did it occur overnight - while sleeping?  Then consider a satin pillowcase, or wear a sleeping cap.  Braiding the hair can also help.

Remember to at least finger comb your hair before going to bed.  If you wear your hair down during the day, finger combing occasionally can help prevent major tangles later.  Try to contain your hair if you are going to be out in the wind.

I know there are other suggestions that others can make - I hope something here helps.

Very very true.  What I do is sleep on a satin pillowcase and scrunchie my hair down(using maybe 4  or 5) and then slip one of my Satin Sox on.  My hair stays exactly the way I left it,no knots,tangles or problems.  I use it when I braid my hair for the night too and it stays the same way as when  I put my braid in when I get up in the morning.