If you need information about how to draft initial correspondence to potential employers (for example, if you want to change jobs or geographic location), please refer to this page for information about student correspondence with potential employers. The same basic rules we provide for students sill apply to a practicing lawyer looking for a change. Review the general guidelines that are discussed on those webpages.
Of course, there are some differences that will apply to someone who has been in practice for several years. You likely will have a lot more relevant experience that you want to emphasize, and you will have to choose carefully what to include in the final correspondence. This will require some thoughtful editing. No matter how much experience you have, it is best to keep the cover letter to one page. Since in these situations your resume can be longer than one page, make your cover letter a sharp demonstration of how your previous experience makes you a strong fit with the new employer.
You may also have a situation in which you are applying to work for an employer you know from your previous professional activities, or an empoyer to whom you have been referred. Don't hesitate to describe the source and extent of your interest in the new employer, in addition to giving examples of how your previous work has prepared you for the move.