Honesty is the Best Policy
Once you’ve made a promise to your clients, you are entirely obligated to fulfill it. But sometimes, circumstances change that affect your ability to go about fulfilling a promise. When these situations arise, be forthcoming with your clients and determine other ways to accomplish the same goal. Be as transparent as you can, and don’t make bold claims if you know they’re impossible to follow through with. We all work hard, but nobody is expected to be a miracle worker.
Make sure you're working for your client. It's important your client feels you are listening to them and actually acting on what they want, not pushing them toward listings that don't fit the bill. More than that, be someone who isn't afraid to tell your client when they are asking for things that are not realistic or possible, like with an offer amount or terms. The honesty factor is huge for clients -- and for future referrals as well.