Clearing hard drive space isn't going to help unless your drive is actually getting full. Less than 10 or 15% free may cause you problems. Other than that, forget about it being a disk issue.
And I can also tell you that repairing permissions will have zero effect on speed except in the rarest of circumstances.
Activity Monitor is going to be your best bet to find if anything is eating unnecessary CPU cycles or memory. Sort by CPU usage first and watch for things acting strangely. The check memory usage if nothing is strange in the CPU area.
Also, look in system prefs under the Accounts section, then click on Login Items and see if you can get rid of anything there.
It would be unusual, but I've heard of people needing to reseat a stick of RAM, so just to be safe, check under About This Mac and make sure it says 2GB.
Mac OS X is not Windows - you should not be having gradual performance dropoffs, culminating in a reinstall of the OS (like I used to do about every 6 months on XP).
If your slowdown is actual and not just in your own mind, there is a reason for it and Activity Monitor is the place to start.