Wiring Practices
Good wiring practices lead to a layout that performs well and is easy to expand or modify as the need arises. Using a consistant color code, running track bus wires near the rails, dropping electrical feeders often, and soldering every piece of rail to either a feeder or an adjacent piece of rail will lead to a layout electrical system that just works.
Color Code
Your layout's color code is up to you. I like a black/contrasting color scheme for track wiring, with feeders for different tracks having different colors. For example, a yard could have three different colors of feeder to indicate arrival and departure tracks, yard lead, and body tracks. Mixing a blue/yellow with a red/black is possible, but does blue connect to red or black?
Alternate buses (such as a 12V accessory bus) could then have a different color code, where the colors employed are not black. For example, a local club uses a blue and yellow scheme to indicate their 12V bus for controlling turnouts.
Some manufacturers or standards organizations have established their own color codes. While I'm happy to use their codes while working on your layout, please make a point of asking for the specific color code.
Track Bus
The track bus on most layouts will follow the track it powers closely. With block control, this results in wires that pass through a section without connecting to anything in that section. On DCC systems, the track bus can be run multiple ways and include things such as autoreverse modules and circuit breakers.
Feeders will always be connected solidly to the track bus, either by soldering or by use of a connector such as an insulation-displacement connector. This will result in a system that can take a bit of abuse before failing.
Feeders
In wiring my own layout, I drop electrical feeders quite often. Every piece of track in my yard either has a feeder attached, or is soldered to an adjacent rail with a feeder attached. This ensures that rail joiners are only used for alignment (and in the case of insulated joiners, insulation) and not electrical transfer.
It is important to note that not all the rails are soldered together. In areas where expansion and contraction must be taken in to consideration, soldering the rail joiners together will not allow rails to move and can cause kinks.
Turnouts
Turnouts will be wired with the intent of being as DCC-friendly as possible. A DCC-friendly turnout has certain properties described at the Wiring For DCC Website (link) and is too complicated to go in to here. Simply put, the DCC-friendly turnout seeks to eliminate mechanical failure as a source for electrical failure.
Converting a turnout to DCC-friendly requires access to the underside of the turnout, so it they're already installed and ballasted, it might not be possible to easily convert it to DCC-friendly.