Agree with everything ah123 said... Never let emotion take over. Thank them for the opportunities you've had with them, and simply say that you're moving to new challenges.
Also, having seen this a number of times, assuming you're giving two weeks notice, continue to do a good job right up until your last day. I've seen too many people start to act very unprofessional during their two weeks saying, "what are they going to do, fire me?". Great way to burn bridges at the last possible moment, even if your original letter was well worded and presented professionally.
Lastly, if you get the first two right, and the situation wasn't totally in the toilet before, ask for a letter of recommendation or referral from your supervisor / boss, etc. These are great to keep on file and use for any future job applications.
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--MR.SKI
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