One of the Agile principles is that software development teams should regularly reflect on how to improve their practices to become more effective. Some systematic approaches have been proposed on how to conduct such a self-reflection process, but little empirical evidence yet exits. In this paper, the empirical results are reported from two XP (Extreme Programming) projects where the project teams conducted "post-iteration workshops" after all process iterations in order to improve and optimize working methods. Both qualitative and quantitative data from the total of eight post-iteration workshops is presented in order to evaluate and compare the findings of the two projects. The results show the decline of both positive and negative findings, as well as the narrower variation of negative findings and process improvement actions towards the end of both projects. In both projects, the data from post-iteration workshops indicate increased satisfaction and learning of project teams.