• The flat part of the needle goes in back
  
• Get all of the fluff out from the bobbin area often.
  
• Change needles frequently (which I never remember to do.. erp!)
  
• Don't pull the fabric though; let the machine take it at the speed it needs it.
  
• Make sure your bobbin is threaded correctly.
  
• Make sure the bobbin case is in correctly, too.
  
• Oil all of the moving bits of your sewing machine that need to be oiled regularly.
  
• Take your machine to be tuned up/looked at/for preventive maintenance on a semi-regular basis.
  
• When using a button-hole attachment on an old sears-kenmore (you know, the big weird-looking greenish-gray plastic thing with the wheel and the hole in the middle of it?) always drop the feed dogs.
  
• If you change the throat plate to the little, straight stitch hole, remember to change it back when you're done, so you don't forget later and try to zigzag something with the wrong throat plate.
  
• Check the tension first.
  
• When you get stuck because the bobbin thread is tangled with the topthread, and you have a huge clump of ickyness, and the needle won't budge, don't tug on it. Try and wiggle the hand wheel to see if you can raise the needle (if it won't work, snip the thread somewhere between the spool and the needle, and unscrew the needle, removing it that way.) Gently slide some sharp scissors under your work, severing threads, and NOT cutting your lovely fabric. Then, open up, get all of the offending threads out of the way, fix your bobbin, check the tension, get a new needle, and try again. (Can you tell this happens to me a lot?)