Good point. The last time I was updating my cover letter, I invented the following workflow:

-copy/paste in the last email template I had used

-start editing sections that are relevant (intro, relevant experience, why I am interested in this position)

-read the entire email as a whole, look for inconsistencies

-touch up any sections that don't make sense, or are weak

-walk away for an hour, do something else

-come back, read it over again, tidy up any sentences, or add any gems I had come up with.

-optionally have a friend read it.

By the end of the process I usually had changed at least 50% of the format, and found that my cover letter was rapidly evolving, until I found out what worked. Sharing cover letters with friends in the industry also helped to root out some weak sections that I hadn't seen myself.

The main disadvantage of this method is that if you're not careful when reading, you can miss a reference to company name you had previously applied to, or include the wrong url. Always, always, always double check any links, names, or other identifying language.