The order in which you discuss these points, the length you devote to each section, and the emphasis you place on particular feature of your background can vary widely from letter to letter. You should definitely tailor the letter to the position. When applying for an adjunct position at a community college, for example, you would highlight your teaching experience and say little about your research interests. For a job at a liberal arts college or university, you might want to say more about your teaching methodology and philosophy. An application to a research university would obviously call for a longer paragraph on your archival experience and research interests and plans.
Find out as much about the job and college before you apply. If the job advertisement is for a British historian, then highlight your experience (in coursework, field exams, research, or teaching) in the relevant fields. It is often a good idea to find out who else is at the institution or its general philosophy--employers can be impressed if you cite something about the school's curriculum, guiding principles, or course offerings (thought be careful not to make this sound gratuitous or fawning). Know the turf that is covered by the other faculty on staff; you don't want to write a letter talking about all the early modern French history you could teach if there is already someone on staff who teaches in this area.
End the letter by noting your availability for interviews. If you are going to attend the AHA, note this as well since it is often the place where colleges do their initial interviewing.
The letter should be neat in appearance; if possible use a laser printer and good quality paper. Most cover letters should be 1 1/2-2 pages long; lengthier letters can be too burdensome for search committees who have to go through many such letters. Check the letter thoroughly for spelling or grammatical errors. This is often your one chance to catch the eye of a potential employer so you want to make the best impression possible. Remember that employers often receive over a hundred applications for a tenure stream job. A lively, interesting letter that highlights your accomplishments is essential to make your application stand out from many others.