Before undertaking a nationwide marketing campaign, spend some time to determine where 80% of your clients are based. You may find that a majority of your clients are based in your own region.
Modify your Web Site to includes keywords about your services in your own region and so obtain a higher ranking in Google when people search for what you do locally. Make sure that your locatin is known to Google so you appear in their maps.
Combine any bulk emails you send to your clients with changes in your web site. Make sure that the email you send contains a link to the relevant detail in your web site.
Perhaps you do not have many client email addresses but you do have records of their phone numbers. You may care to ask such clients if they may have an email address they would not mind sharing with you so you can provide details electronically rather than by post. Some of those calls wil result in new business because it may cause the client to remember there is something they want you to do for them.