Write numbers below 10 in word form. So, instead of "There are 5 people in the room" you should write "There are five people in the room."
Write numbers for 10 and above in numerical form. This is to make the flow better -- rather than "There are five thousand, six hundred and fifty-four people in this tiny town" you should write "There are 5,654 people in this tiny town."
Use decimals for numbers over 1 million. Again, this improves flow -- 5,600,000 takes up a lot of space and requires the reader to count zeros; 5.6 million, on the other hand, is easy to read and understand and takes up less space.