The Lord appeared to Abraham in Genesis 18 in the form of three men. The men again promised that Abraham would have a son. This time they gave an exact time - "this time next year." Perhaps for Abraham,the promise still seemed a bit far fetched.
Sarah certainly thought it was. When she heard the promise she laughed out loud. She was well beyond the age to be able to have a child so the promise seemed ridiculous. She didn't believe what the men had to say . . . she didn't believe in what God had to say.
Let's be honest - there was NO REASON for Sarah to believe what God had to say on this matter. It's not like Sarah could point to a track record of God's good works in her life. If you read the chapters leading up to the story you realize that Sarah had it rough: moving from one place to another, being mocked by Abraham's concubine, and unable to have children. The work and promises of God were scant in her life. She really had no reason to believe in that promise. We certainly would not have (no matter how much we use this passage as an example of what not to do).
The reality is that we have a much larger track record than Sarah did of God's work in our lives. We have no reason to doubt or laugh at the promises of God. The greatest example we have is the person and work of Jesus. God coming in human form is evidence of God's love for us and His intention towards us. In Christ we have the eternal "yes" of God. All of the promises in the OT are confirmed in the coming of Christ.
God gave you a "yes" . . . believe the promises because God has confirmed the promises in Christ. He has a track record in our lives!