1 Omit "I think". I've run into this error a lot, especially when proofreading college students' papers. We know that's what you think. Saying so weakens your argument.
2 Don't use two words when one will do.
3 Use adjectives to add punch...
4 But use them sparingly. The right adjective can breathe life into your writing... [but] two adjectives to a single noun are usually too many, and even one is risky (source).
5 Use sentence fragments. Occasionally. But, be sure to use them only in certain situations, and for emphasis.
6 If the piece you're writing is informal (say, a blog post or a penpal letter, rather than a business letter or a college essay), use contractions. Doesn't 'I'm having a party and you're invited" sound friendlier than "I am having a party and you are invited"?
7 Be concise, always.