Can people who suffer from motion sickness become pilots?
The short answer is yes. I can't answer all your questions in detail, but here's some US-based information.

First, is it disqualifying? According to the FAA, if a candidate for a medical says he suffers from motion sickness then "a careful history" is required, and if the person requires medication then FAA approval (i.e. a special issuance) is needed. So it all depends on the person's individual situation, but it is definitely possible to get a medical, even if you require medication. Motion sickness also seems to be fairly common in new pilots, by the way:


  Approximately 10% of all students taking flight training will become
  airsick at some point during their first 10 flights; 15-20% of these
  will have a severe enough form to interfere with their control of the
  aircraft.


Second, are there alternatives to medication? The FAA has a video about motion sickness and how to deal with. I haven't watched it but the description implies yes:


  This video provides practical advice on diet, medications and other
  exercises to help cope with this potential problem


Wikipedia also mentions various treatments although some (like head-mounted displays) seem impractical for civil aviation at least.

Third, do pilots adjust over time? Apparently, yes. This FAA document says:


  The experienced or acrobatic pilot is conditioned to withstand abrupt
  attitude changes that a passenger or helicopter pilot might not have
  experienced.
  
  [...]
  
  Continued gradually escalating exposure is the most common process for
  facilitating adaptation. Several days and repeated sessions of flying
  usually conditions the patient to the new environment.


That exposure would also be an alternative to medication, of course. And Sami's answer has some interesting personal experiences.

Finally, are pilots taught techniques to stay focused on flying? Instrument rated pilots are trained to ignore their physical sensations and focus fully on their instruments instead. But that training has nothing to do with motion sickness as such, and I have no idea how pilots who do suffer from motion sickness are affected (or not) when flying on instruments.