It has not been my experience that Macs get too crufty, but there are some things you can do to try to clean them up.
As Fortyrunner said, disable any login items you do not need.
Open System Preferences and search for ‘login items’. That will bring up the login items tab of your account in the Accounts preference pane. If you see stuff in there that you no longer want, select it and click the minus button to delete it from the login sequence.
An alternative is to create a new user and start from scratch.
Many users seem to swear by doing a regular “cache cleanup”. This seems counter-intuitive to me. If you want to reclaim disk space, then sure clean those caches. But otherwise caches are supposed to make things faster. I know there have been some bugs where some applications suffered slow lookups when its cache became very large (I think Safari had this problem with its favicon cache at one point).
There are tools to help do this. Usually their cache cleaning functions are bundled together with other maintenance tasks. In theory, caches can be easily removed by hand (most of them are stored in
~/Library/Caches), but maybe the tools add something extra beyond “push button” ease of use.
Here are some tools I see referenced fairly often: OnyX, Cocktail, AppleJack. Some new ones I found while researching: Cache Out X, Yasu. Most all of these have other maintenance functions that you might also consider.
To some extent, modern HFS+ volumes will auto-defragment files (see “On-the-fly Defragmentation” in Fragmentation in HFS Plus Volumes). But some work loads might benefit from free-space defragmentation or other on-disk optimization. Your video/media editing seems like might benefit from free-space defragmentation since you probably deal with very large files fairly often.
If you are interested in this extra defragmentation, you could try a defrag tool (iDefrag, the defrag tool in Drive Genius, the “optimize” tool in Techtool Pro, maybe others).
These tools all cost money and are not exactly equivalent. The only one I have any personal experience with is Drive Genius. I can not really say whether it made anything faster because both times I have used it I was just testing its defrag function on a freshly restored volume on a brand new drive.
Fortyrunner is right-on about RAM.
When looking for a speed boost, always take a machine up to its maximum amount of RAM (or as much as you can afford to put in there).
After RAM, you can think about upgrading the hard drive. I notice a speedup when I run off of 7200RPM disk (external SATA behind FW400) compared to when I run on a typical 5400RPM PATA internal. If you only have a 5400RPM drive in there, you might look into 7200RPM drives (just try not to get one that uses much more power than the original, since it might “cook itself” if it puts out more heat that the computer is designed to dissipate; if the current drive is older, it is fairly easy to find new, faster drives that have about the same power consumption).
As Fortyrunner indicated, it is generally held that newer major Mac OS X releases almost always run faster than their predecessors even on the same hardware. If it is an Intel Mac, you might consider buying Snow Leopard.