My research area is centred on AI planning, with the current work focused on optimising low quality plans generated by existing planners. Since the beginning of my PhD studies, I have concentrated on studying existing plan-optimisation techniques. Efforts have been made to compare, evaluate and test them on several benchmarks domains and planning engines. From my work, I have concluded that adapting different techniques to work together is pivotal to establishing an efficient technique. Besides this, another very important aspect to study was the theoretical properties of these techniques, including completeness and complexity.