The first and best option you can have to safeguard yourself from customers that won’t send or approve artwork is to have a line in your contract or agreement that states billing will start regardless of artwork approval. Usually, something along the lines of “Advertiser has 2 weeks to provide and or approve artwork. Billing will start regardless of artwork approval”.

If the slow client is a digital billboard customer, you can design a very simple and generic “logo ad” and email it to them with the verbiage “We will start with this, if you want any changes please let me know”.

See the example below of a very generic billboard ad.

Generic Billboard Idea