Well there is not much you can do to make Photoshop run faster or smoother. But... There are many things you should do to make life easier for yourself that will speed things up a little or a lot.

1. Try to have as much as 2 to 3 times the amount of actual RAM vs. the actual size of the file in Megs....e.g., for a 50 meg file you should have 100 to 150 Megs or Physical RAM allocated. If not you will start to use the scratch disk. This is where you will see significant slowdowns. No, Virtual Memory does not count for physical RAM. I have 600 Megs at the least allotted for Photoshop and up to 1.5 gigs when needed.

2. Use as few layers as possible. As long as two items are not touching they can be put on the same layer. The reason I say this is because if you have a 50 meg file with 50 layers, it will take a lot longer time to scale, rotate, distort, and apply any filters than it would if it had 5 layers. Obviously you can't always get rid of layers, but if you need to apply a very complex filter, then make a duplicate of the file and try compressing the heck out of the layers. Experiment - you will be surprised.

3. The next best trick I use a lot is actions. I make actions for a lot of the things I do. Example: I need to make shadows for a catalog I do. So I make an action for it as follows.

First thing I do is get a selection the best way possible (I could teach you close to 10 ways to make a selection), then the action is this:

Contract 1 pixel (so it traps), Make new layer, Apply selection as mask for that layer, Load mask as selection, Inverse selection, Select the marquee tool, Nudge selection X by Y inches down and right, Feather 8 pixels, Fill with 100% of foreground color (0, 0, 0, 100, is best color for shadows, not a 4 color black), Change opacity of layer to 40%, and Multiply layer.

All of that is done in about 4 seconds on my G3. It would take me close to 20 seconds to do it all by hand at least, and when you are busy working sometimes you get distracted and it might take a minute. And this way you can not make a mistake. They are all uniformly built. And if I have to create 200 shadows a day, it would save me close to 20 minutes.
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Thanks to McKs for the additional tips below! Cheers :-)

While a Mac can handle up to 1.5 Gb of RAM, you can ony allocate 999MB per app in Photoshop.

Also, seems to like it when you set your disk cache as low as it goes.

Turning off automatic creation of a €rst snapshot also helps, especially if you create many new docs (i.e. from history states).