like how do you know what new component is coming out
Announcements, and occasionally leaks, from the manufacturers of the components. This is what you need to know to plan around - for example, you could buy an Intel i5 2500K (a Sandy Bridge series CPU) at the moment. However, this probably isn't a good idea since Intel announced quite a long while ago the next series (Ivy Bridge) will be available in Q1 of 2012 (i.e. the first financial quarter), which means 'within the next couple of months.'
Computer hardware doesn't depreciate in value very regularly - not as regularly as cars, anyway. Some components, such as hard drives, depreciate much more slowly than others, such as GPUs (graphics cards). This is because if you bought a 1TB hard drive a couple of years ago, it's only going to be marginally slower (<1%) than a new 1TB released today, and not a whole lot cheaper. GPUs, however, depreciate fairly quickly as soon as the next series is out, and so within a couple of years what was high-end GPU can drop in price by at least half, since it's equivalently powerful to a new mid-end card and has older technology as well.
There's also the occasional breakthrough technology which can drastically reduce the prices of the older stuff. A notable example right now is that IPS screens (screens with excellent colour reproduction, are normally a luxury product) are becoming a lot cheaper, which in turn is also driving down the price of the lower-spec standard TN screens.