Writing stable worlds, as of 0.10.4, is really simple: every world starts stable. Adding some supplemental features, including both terrain modifiers (such as caves) and populators (such as villages and mineshafts) will reduce random additions, which in turn reduces the likelyhood of random instability being added by accident.
For 0.11 / 1.7.10, the most important thing that affects stability of a world is the accessibility of ores. As a rough approximation, this means that the more air, and less solid ground at a given Y level, the more valuable the ores at that Y level are considered to be. This means that adding lots of caves, or lots of mineshafts, by making more air passageways to expose ores will increase the instability of the age.
The following is for earlier versions of Mystcraft.
The most important point in writing a stable world is to include one each of all the primary symbols, excluding biomes. The biome controller (of which there can only be one) determines the number of biomes. “Single Biome” obviously takes one; checkerboard currently only takes two. “Native Biome Controller” is the oddball, and takes ZERO – it uses the internal list of biomes used in the Overworld.
Beyond that, the big question for the would-be age author is what is considered primary? The simple question to ask yourself, as a writer, is: “Would this age make sense if nothing from this category was included?”
For example, a world with no time signature at all would not make sense. Whether it's a single fixed time (a world that is tidal locked with the sun), or a world that spins faster or slower than normal, there needs to be some concept of the world's spin, or time. Hence, time is a primary symbol in 0.9.x, and a sun (note that a dark sun counts) is primary in 0.10.1. (As of 0.10.2, a world can have 0 or more suns without issue).
This same approach can be used to examine the other categories, to determine which symbols must be included for a stable age.
In general: If a symbol is stackable, it is not primary.
A blank book is one that has no symbols. Such a book can be used, but is guaranteed to be unstable as all primary symbols are randomly added by the game.
If you want a random world to explore that is stable, use the administrator commands
This will give you a random world, stable (as of 0.9.4 and later), and a book for it.
Some symbols are rarely encountered normally, and are a sign of a greedy author. If you have written your world such that crops grow faster, or such that there is more ores than normal in the ground, expect the world to have some instability, including various forms of colored decay
A word of caution to the really greedy author: You may have a world with very large amounts of ore, that will be eaten up in front of your eyes. Assuming, that is, that you can even see at all.