(Note to people who haven't used mystcraft in a while, the method for getting symbols has recently [version 0.10+] changed from randomly generating worlds to finding symbol pages in the overworld, see this wiki page for more details)
From the mystcraft wiki page on writing stable ages the basic approach you need to take to is:
The simple question to ask yourself, as a writer, is:
“Would this age make sense if nothing from this category was included?”
To write a stable age you need to ensure that at least one of every category of primary symbol is included in your descriptive book, there is not AFAIK a definitive list of primary symbol categories.
Here is a list of symbols from the Mystcraft site that explain their functions to some degree.
Direwolf20 has a mod spotlight that covers all the details of how the new writing system works
Primary symbols are those that specify basic properties of the age such as time flow, lighting, biome, etc; in short any feature of that you would expect a 'real' world to have.
As Dan says there are also some symbols (Dense Ores being the most common example) that are guaranteed to introduce instability into the age.
If you want to practice writing stable ages then you can generate a new creative world which will make a 'Creative Spawned' notebook available that will contain all symbols automatically. Using a separate world will also allow you to easily avoid any problems from bloating your save game with a large number of unwanted worlds