A few other things that help which have been mentioned elsewhere on the forums:
1. try disabling sound ( in pslaunch / settings )
2. If you are tight on RAM, try closing any other open programs that are not needed while playing ( web browsers, outlook, word etc .. )
3. If you have a virus scanner, make sure its not scanning while your playing ( common sense, but ive see this bite people in the rear numerous times )
4. check your system resources to see if you are running out of ram. ( for windows, right click the start bar and then "task manager" or else run taskmgr.exe )
-- Ignore the fact that it shows CPU %100 utilized. At time of writing this game seems to use %100 cpu for which ever core its running in ( assuming you have a multicore cpu ). I suspect this is by design to eliminate latency due to process switching ( something like a spin-lock )
5. ensure that your using a driver that takes advantage of the video care for acceleration. You want to ensure that rendering is offloaded to the gpu.
-- for linux, check that opengl is installed and your using the right driver ( for me, its the proprietary nvidia driver ) - I get a very significant performance increase if I use this driver instead of the generic open source driver for nvidia. For windows and mac this should not be an issue all.
6. If your running on linux and are short on ram ( 1g ) try using a lightweight desktop manager - gnome and kde are seriously bloated. Try xfce, blackbox, fluxbox etc... xfce has the feel and functionality of a modern desktop while using up very little ram.