There’s nothing wrong with gushing praise for your best customers.
Praise them, thank them, and most of all: Give them STATUS.
Nothing motivates like public praise. Reward good behavior and you’ll get more of it. Reward good behavior publicly and you’ll have folks eager to do what you want.
The grand champion of this is eBay. Take a look at the email example below.
How can you make your customer look great to their friends?