The elements in the flag should be so simple that they can be drawn from memory with a box of crayons. Just unique enough to be memorable, but simple enough to be recreated by anyone from a small child to a skilled illustrator.
Remember that flags are made to be flown and need to be identifiable when battered about in high winds or when hanging limp in calm. Use a flag simulator to test the durability of your design.
Only create a unifying visual concept that all the elements of the flag support. If possible try to write done a single word that your flag represents. This could be a specific idea, like nature or an abstract concept like stability or precision. Make sure all the elements support this idea, whether through literal shapes or symbolic colors.
3. No More Than Three Colours
This isn’t a hard and fast rule, but it is generally good practice to use only a few colors. For better contrast, in most cases you should use primaries from the Opponent Process: Red, Green, Blue, Yellow, Black, and White. There is a standard color set that almost all flags use, and should be followed in the vast majority of designs.
4. No Text, Coats of Arms, or Seals
A flag is a thing in itself; don’t just take an existing seal or coat of arms and place it on your flag. These designs are often visually complex with far more elements than belong in a flag. Text especially does not belong on a flag as it cannot be read easily at a distance or small sizes and requires that the viewer be able to read that particular language.
Avoid confusion with other flags and be deliberate about introducing significant similarities between your flag and others. For instance, if you were to include a cross similar to this one:
you should be aware of its strong association with Nordic countries. This may or may not be appropriate for your flag depending on the heritage or philosophy your organisation would like to espouse, but it is certainly worth being aware of.
You can read more about these principles in Ted Kaye’s Good Flag Bad Flag or watch Roman Mars’s Ted Talk Why City Flags May Be The Worst Designed Thing You’ve Never Noticed. He also made a list of flags that are successful despite breaking some of these rules. Remember that these are just guidelines, and while they are a solid foundation on which to build, once you understand the underlying principles you should feel free to break them wherever appropriate.