Thinking about a "proper" install?

Just something about sound proofing.

It doesn't necessarily mean use dynamat all over the shop. Dynamat's good, don't get me wrong, but stuff like expanding foam can be used too, to fill in little gaps between panels, etc. It's also a fair bit cheaper.

Installation is important. The sub enclosure needs to be well built, completely sealed (apart from where it doesn't need to be, e.g. ports) to get the most of it. Some prefabricated boxes really aren't up to the job IMO, and they may not necessarily be of the correct volume for your sub. if you build your own, then you can decide on how thick you want to make the enclosure walls, bracing, shape etc. The speakers also need to be solidly mounted to cut down on unwanted resonance. This will give improved sound quality.
Make sure your amp/s are cooled sufficently. DO NOT MOUNT THEM UPSIDE DOWN (e.g. on the underside of the parcel shelf). Amps produce heat, which it dissipates. Heat rises, so if it's upside down, the the heat would be radiated out, only for it to rise back into the amp, causing overheating and possibly destroying the amp. which is not good

As Paul said, cabling is important, and that every part of your install gets the power it needs.

Well said!
However my installs are always such a part of the car that I can never bring myself to remove them when on-selling. The dashboard of my 164 would never have looked the same without the upmarket Blaupunkt London RDM168 in there with its piano-black finish, and the X1/9 just wouldn't have been the same without the blue-and-red illumination of the cheap(er) Blaupunkt Miami Beach CD51. I'm beginning to lose count of the number of Infinity speakers that I've bought, too. Still, there is always another great choice available for the next install. And I'm sure the owners of my previous projects are appreciating the sound quality...

I think the best example was my 'first' car (actually my second, but the first that I had been responsible for), a 1982 Austin Princess that I totally rebuilt at a cost of over $5000, reconditioned automatic transmission, rebuilt engine, full respray in British Racing green, complete with 12" subwoofer boxed under the rear shelf, component speakers up front, two amplifiers and gold-plated power connectors. The stereo equipment was worth $1200 alone, and without it the car would have been worthless. With it, the car sold for... $1200.

-Alex

All advice is good advice.

The best advice I can give from my experience is defainately running the RCA and power/remote wires down opposite sides of the car to decrease interference.

Another handy tool is the ground loop isolator, get sweet signals = sweet sounds

Enjoy all!