Trust management is one of the most challenging issues in the emerging cloud computing. Although many approaches have been proposed recently for trust management in cloud environments, not much attention has been given to determining the credibility of trust feedbacks. Moreover, the dynamic nature of cloud environments makes guaranteeing the availability of trust management services a difficult problem due to the unpredictable number of cloud consumers. In this paper, we propose a framework to improve ways on trust management in cloud environments. In particular, we introduce a credibility model that not only distinguishes between credible trust feedbacks, but also has the ability to detect the malicious trust feedbacks from attackers. We also present a replication determination model that dynamically decides the optimal replica number of the trust management service so that the trust management service can be always maintained at a desired availability level. The approaches have been validated by the prototype system and experimental results.