Three principles will take you a long way towards writing clearly:
Judgement is always required, but the three short rules are an excellent starting point. When in doubt, keep it short.
Which is the most important of the three short rules?
That's lucky, because sometimes you need to use long words. How would scientists, academics and politicians function without them? And life would be dull indeed if all words longer than two syllables were unacceptable.
The more long words you use, the more important it is to use short sentences.
EXAMPLE OF PLAIN LANGUAGE: LONG WORDS + SHORT SENTENCES:
Internationally, governments are legislating for plain language. The plain language bill now before the US Congress and Senate is an influential prototype. This looks set to become law in 2009.
EXAMPLE OF HARD-TO-READ FANCY LANGUAGE: LONG WORDS + LONG SENTENCES
There are many examples of governments internationally mandating plain language in legislation, the most influential contemporaneous prototype being the plain language law currently before both the US Congress and Senate, with an intrinsic probability of enactment in 2009.