Do your homework.

The first thing to keep in mind are the proximity effects. If you have conductors parallel to each other, carrying time varying signals, they will induce a time varying magnetc field on each other, which will induce a voltage in both - this stray voltage is not wanted.

Read for example http://www.solidodesign.com/uploads/1_drennan_cicc06_v3f.pdf - This does not really talk about things in an easy DIY manner, but it helps to read it.

Then you might want to read the rules of thumbs for keeping wires away from each other - such that they are not influenced by each other . Make sure that you read the relevant documents, such as this .

As a rule of thumb : Twist wires that carry signal in different directions at the same time together to minimize this.

Secondly, each pair of conductor in the Universe has some capacitance between them. Dealing with it is a bit more complex than the previous point.

Here is a good document to prepare for PCB design issues.