You should just keep it brief, say that you feel it's time that you move on to new pastures, and that you value the experience gained while working in the job.
Remember that you might need a reference from them in future, so do try to be diplomatic!
Not knowing what you experienced or thought of the employer it is difficult for another to say. Why not just be honest ? Worried about references maybe ? Or not wanting to burn bridges behind you ? Have to say I've always been honest in a resignation letter. Hoped it might help those I left behind.
I resigned from a job I did not like just 6 weeks ago. I think I managed to stretch the one sentance to two! It worked, I am now happily ensconsed in my new job!
Whilst I have enjoyed my time at xxxxxxxxxxxx, I feel it now time for me to pursue my career elsewhere. Please accept this letter as my resignation from my position as xxxxxxx, giving the required xx weeks notice, making xx/xx/2013 my last working day with xxxx company.
I would also like to take this opportunity to thank you for the opportunity of developing my skills during my time here.