There's a long-standing tradition in sales that's about as self-destructive to a sales team as it's possible to get: Treating salespeople differently depending upon their sales metrics.
You all know what I mean - making exceptions for things you would never normally allow because a particular salesperson generates high numbers. While it's great that they are consistent achievers (wait - they're not consistent? then why are you rewarding them for sporadic results?), think about the door that you've opened. These are salespeople we're talking about; it's their job to open doors as wide as possible, and you can't expect them to act differently with internal doors than they do with external doors. They will push those exceptions as far and as wide as they possibly can, and that's not their fault - that's what salespeople do, by nature, and you're the one that opened the door.
Now think about the other members of the sales team, the folks who consistently hit goal but don't soar above it. How much of a motivator is it for them to see that the fellow who occasionally hits a high note is permitted to break the rules, sending their sales even higher, while they have to rigidly observe procedure?
It's not. That's why they're sullen. That's why their results decline. That's why they leave, and go do terrific work for your competitors, taking knowledge about your company, your Customers, and how you do things with them.
There's nothing wrong with compensating salespeople based on results - that's what sales is all about, and everyone understands that. It's the arbitrariness that makes unspoken benefits evil, not the concept. If you're going to bestow privileges based on results, put them down in black and white, for all to see and aspire to. And, rather than have them kick in when a salesperson goes above and beyond just once, make them effective with consistent high results.
Salespeople are like racehorses: It's their job to run, but it's your job to clearly lay out the track and the stakes.