Since approx. one year I'm using a SSD in my Mac mini. After having it built-in I remember a speed-improvement compared to the default hard disk, but today it (the Mac mini) feels sluggish. What can I do to improve the SSD performance, except of reinstalling OS X 10.6?
You probably have an SSD with severely degrading performance due to OS X's lack of TRIM support.
There's nothing you can do, except erasing (this can be made less painful e.g. by using the OS X Installer's Time Machine restore capabilities) or get a different SSD.
Also, you can use dtrace to get some insight into how your disk is being used. For example,
As you can see, this shows you histograms of your disk's transaction queue. If you have a peak somewhere else than at 0, that means your disk is pretty busy. You can also use the iotop command (with sudo) to see what processes are using the disk.
It's not going to make your SSD faster but at least you have some idea of what's going on...
Oh and to simulate TRIM you can try running "Erase Free Space" from the Erase tab of your disk in Disk Utility. That will overwrite free space with zeroes and should have the same effect as trim.