Be your own customer.
To the extent that you can be a patron of your (or your client’s) product or service, the likelihood of improving that product or service increases, often substantially.
Allow me to examplificate: At nearly every conference where I’m a speaker, I give away one of my letterpress posters at the end of my presentation. Instead of carrying on the airplane the 6"×24" tube containing the poster, I often ship the poster using the same carrier I use to ship posters to customers. This allows me to experience first-hand how the poster arrives and survey any damage that might have occurred during shipment.
Another example: Whenever I’ve needed freelancers for Authentic Jobs or have made a full-time hire, I’ve posted a job listing using exactly the same process as our customers. Occasionally I post on competitors’ sites, too. I find it’s a terrific way to eat our own dog food and examine the food of our competition.
One more: Years ago at a small dotcom agency where I was the creative director, Tupperware was one of our most important clients. I had little experience using Tupperware, so I registered as a independent rep and began selling product online and servicing customers in my area. On a particularly interesting evening, I found myself attending a live event for reps, with 80's rock anthems blasting from the PA system and lots of excited women clamoring about plastic. It was a unique experience, to say the least. (Truth be told, I enjoyed working with Tupperware more than any other client at the time.)
In all three cases, I’ve gained valuable insight for improving the product or service I was responsible for. True, I‘m unable to replicate the customer experience perfectly, but I can certainly try. And the trying has clearly paid off.