1. KEEP TO THE WORD LIMIT
Universities often provide a word limit or a page limit to guide you. Keep to it. If they say they don't want to read three pages, they mean it. Bear in mind that academics have to do an awful lot of reading, not only of statements of purpose but also of essays and theses. If you can't keep to the word limit for a statement of purpose, they may be worrying that you will write a 450 page thesis when 150 pages was the limit. Writing too much is never a way to make yourself popular. If a limit is given, it is good to set yourself a personal maximum limit of 10-15% less than that. And don't feel you have to fill a word limit. If you have said all you want to in 700 words and the limit is 1000, great! Stop. Don't go looking for verbiage to pack in the spaces.
2. SET YOURSELF SECTION WORD LIMITS
If you have 800 words, have in mind how many you want to spend on each section of your statement. If you use 750 words describing your studies to date, you will have nothing left for the other sections. By setting yourself rough word limits for each part, you ensure that the statement is balanced.
3. BE SELECTIVE
With any piece of writing where there is a word limit, you will not have enough space to say everything about everything. This means you have to be selective. You have to gather all the necessary information, look at it and throw away the things which are less necessary. It may hurt not to be able to say that you got top grades in your school for physics (when you're applying to study sociology) but you have to be ruthless. Remember that the ability to evaluate and select what to include and what to leave out is valuable academic skill in its own right, and demonstrating that you have that skill can count powerfully in your favor.
4. USE APPROPRIATE LANGUAGE
Obviously you need to show you have a good command of the English language: avoid slang, use vocabulary appropriate to your field and show that you can write a sentence of more than 5 words. At the same time, don't start looking for long words to impress with. If an ordinary word will do, don't go thesaurus hunting for a bigger one, not least because you may use it wrongly.
5. EDIT THOROUGHLY
When you have written a first draft, go over it and check whether any of your phrases are wordy or clumsy. Try to re-express them clearly and succinctly. While it is good to use longer sentences sometimes, don't ramble. If your sentence has more than 30 words, read it over and see if it would be better to split it into two. Reading aloud may help you to feel if your ideas are clearly expressed.