The Plain English Campaign advises:

average sentence length of 15-20 words

plenty of 'active' verbs

words like 'we' and 'you' rather than the patient or the organisation

conciseness

clear, helpful headings with consistent and suitable ways of making them stand out from the text

good type size and clear typeface

average line length of between seven and 12 words

for forms, a logical flow and plenty of space for answers