It's all about vectors masks and layer styles. Wanna make a sweet glossy button? It's just a simple rounded rectangle vector mask with a gradient fill, drop shadow, inner/outer glow and texture overlay.

Also learn about blending options/modes (i.e. screen, multiply, overlay, etc.) for combining effects.

Also subscribe to the RSS feed of 365psd.com, download each day's psd and figure out how it works.

This. Photoshop has plenty of filters for interesting effects, but layers, in my opinion, are where some of the meat and potatoes sit. A lot of times I find myself going back and forth on style changes, such that I was relieved to find the layer styles feature for myself.

I also make heavy use of solid color layers; the color layer is a layer composed of two things--the layer of color (which extends into, conceptually, infinity) and a mask which helps in managing of the area through which the color shows. The great thing about this is that you can change the color of the layer at any time, over and over. You can do something similar with layer styles, where you choose a color overlay for the layer, as well.