What is the best way to get new customers for a digital agency?

This is one of the core reasons agencies are a service-centric model vs product-centric: quick to monetize and a function of human labor/ effort to scale (unlike products that are usually slower to monetize, require high initial investment but scale really well).

So the answer is either you need to outsource sales and lead generation to an external firm, use a platform like Clozer or inhouse the sales folk. When inhousing sales: #1 recommendation: don't make the sales people find their own leads. They hate that. You need a lead generator (could be outsourced) and a sales person.

The leadgen should be supplemented with inbound marketing for the best effect. The function of a dedicated "leadgen" person is set up meetings and should be rewarded on successful meetings.

First thing you ask the sales person: WHERE'S YOUR BOOK? ie:who are you going to bring with you today (clients) that we can close quick based on existing relationships.

Good sales teams and systems are costly - so chose a path that monetizes quick.

Greg has the key elements. Now since you don't have Business Development resource in-house you are going to have to hire or outsource to get the skillset you need.

Before that, and the most important part of the CVO is "Determine your Product / Market Fit". That's the core and here I strongly advise you spend some money on hiring a marketing expert who can facilitate a session with you to extract that, capture it into a written strategy plan and help you write the tactical implementation plan. (that's the Return Path on CVO).

Then you have the core already agreed and can use this to brief outsourced folks to write landing pages, ad copy, research DM lists, run social, do follow up calls etc. In this way you can hire execution people who are cheaper than strategy people.

My own Lead Generation Methodology follows a similar pathway detailed on this slideshare deck

The Art and Science of Generating Leads
http://www.slideshare.net/rcaroe/the-art-and-science-of-generating-leads

The CVO or Customer Value Optomization model (it goes by many other names as well) but for a business software development company combining it with a B2B Viral Loop is going to be how you achieve breakout growth. That and find more partners like the one that is currently referring you so much traffic.

CVO Model:
http://www.digitalmarketer.com/customer-value-optimization/