Get yourself a copy of IPC-2221 Generic Standard on Printed Board Design (previously called IPC-D-275). My copy is printed in February 1998. Published by the Institute for Interconnecting and Packaging Electronic Circuits. It is 98 pages, very readable and well-organized. It has all sorts of good stuff like trace width, thickness, spacing, component connections, etc. For just trace widths there are several online trace width calculators provided for free by some circuit board companies, like AdvancedPCB. On their website the even cite IPC-2221 as their source.

In addition to these consideration there are also application-specific considerations having to do with high frequency signals and noise pickup. For example, a trace carrying an Radio Frequency signal cannot be too long or the trace itself will start acting like a transmission line.