Let me start by saying I own a marketing agency, so I feel you on this.
First of all, when they come back to you wanting more work, ask them for a testimonial if they didn't give one before, ask them for recommendations on LinkedIn, ask them for reviews on your FB page and Google Business page and remind them for referrals. Send them postcards or flyers to hand out to friends or other nearby businesses. It's also important to not ask people to do too many things in these email blasts, so make sure each email is only asking them to do one thing, such as review FB or refer a business.
We successfully send out individual emails (not email blasts) to our clients once projects are finished and ask them to review us on Google, and provide instructions on how to do so. Between 25% to 50% of them follow through, which is great.
You obviously want to have (and maybe already do have) a referral program where you give clients a discounted or even free service for any referrals that turn into customers. Be sure these people know about that.
For other ideas, we have to think outside the box. Some ideas include:
-Send customers an email about an eBook or a white paper or inforgraphic that would be really helpful and beneficial for them to read, and have the lead capture form include fields for the contact info of another one of their business-owning or marketing director friends, so then they get the ebook or white paper, as well.
-You can make a video tutorial or even host a live video on FB about how to do something in Photoshop or how to make a Case Study in Word or something easy that this audience could do, and tell them to share it with their friends in the industry, since it's free helpful info.
-Similarly, you could host a webinar or even a live event in which people have to sign up and give you their email addresses, so you can ask your customers to inform their friends and peers about this event, which can give you more leads.
-You could even hold a contest and send out an informational and promotional email about yourselves to your customers and ask them to forward it to their peers or nearby business owners, and to BCC you guys on the email, and then after a couple of weeks, you will randomly select one customer who did this and send them a $100 Visa gift card.
These were just some unique ideas I thought up. We get a ton of business from refferals but you also want to make sure you're doing other lead generation tactics to expand your reach and drive more people to your website. Best of luck!!