If you want build a custom system and not use some lightweight distribution, you have to do some more research. One of the sites I have found most useful, is the Gentoo wiki. It has been most helpful also at times when I have used some other distribution. For example, if you want to learn to customize some lightweight window manager, you will probably find all the best tips and tricks there. One good example is the page for tweaking Openbox window manager. Most of the information can be used with just about any distribution.
Other tips are probably more obvious: If you are using any Debian based distribution, for example Ubuntu, you should study the Debian reference. It has been translated into many languages. Slackware Linux Essentials - The Official Guide To Slackware Linux is similarly useful for those who use a distribution based on Slackware, for example Zenwalk or Vector Linux.
If you read the fine manuals, you will be able to solve most of your problems. If not, you can always ask in the Internet forums.