How do I tell professors I am no longer applying to graduate school?
You have a number of choices at your disposal:


In person. I would recommend this as it offers the most rich (in terms of feedback) setting for you to explain your situation, offer your gratitude for their help thus far, and secure the notion that you would be revisiting your graduate applications down the road given your new job. However, given the fact that you may not be near your university, this option has its drawbacks.
Through a Phone Call. Second to an in-person conversation is a phone call. You can receive more feedback in terms of their reaction to the news through audio cues. At the same time generating a more organic flow to the conversation.



  Now I'm at a loss over how to tell my professors I'm putting a halt on my applications. I feel shitty for making them write letters that I won't use, and worse, I know I will be asking them again in a few years to apply again.
  
  Is there a polite way of emailing them not only do I not need the letters anymore, but to also hang onto them because I will need them again?


As for email, here is something to consider:


  Dear [Professor X],
  
  I wanted to thank you for the time and effort that you have put into
  writing a letter of recommendation for my graduate studies. I really
  appreciate the help and advice you have given me through the process. 
  
  I recently received an offer to work at [insert company here].
  Although I had not considered it before, this position would allow me
  to [insert reason here]. I have discussed this with [potentially other
  professors, significant other] and I decided that this would be the
  best move for me in the next couple of years.
  
  This does not mean that I do not have any aspirations to attend
  graduate school however. I understand how you may be disappointed in
  my decision, but I am hoping that you would support me if I were to
  postpone my graduate studies and reapply down the road. I am wondering
  if you would still be inclined to help me.
  
  Please let me know of any comments or reservations you may have about
  this. Thank you.
  
  Respectfully Yours,
  
  [Your Name]




As for your questions:


  I can't tell if my email should be a be a casual heads up or a weighty explanation of how I fucked up.


Keep it simple and to the point. You did not fuck up, you are making the best (relatively speaking) decision for yourself at this point in time. 


  Is it common for people to give up on their applications like this?


No hard data, but people quit programs for a multitude of reasons. You deciding to pursue a position is not the end of the world nor a unique situation.