Some of my thoughts in bullet point form, some of which have been covered. That's all good advice above by the way
1. Pick a topic you know well
2. Make your text and reviews honest and not promotional. If a casino does something not quite as good as you'd expect (ie: slow payouts), tell the player. Best they make the decisions while they are on your site rather than be disappointed after you've lost them! Which leads me to...
3. ...manage player expectations. Don't over-hype a casino. Best to under-hype them and have the player get more than he/she expected when they get there. Balance!
4. Don't over-rely on bonuses selling it for you. The WR can often contradict (3) above. Unless you tell them the WR first of course
5. Learn the basics about SEO
before you start
6. Get to the point as quickly as possible in your text.
7. Send players to good casinos! Don't be swayed by affiliate $$. 10% from a top notch casino is potentially far more rewarding than 100% from a crap one. A happy player = a happy affiliate.
8. Understand your audience. If you are aiming at a High Limit Blackjack player, then advertising a $100 bonus is not likely to be a selling point. If you're going for new UK slots players, then "slot machines" is better terminology than "bonus slots". Etc.
Too many online businesses think about the "business" before they consider the "customer". Bad business. If you can get the player what they want, then you'll more than likely get what you want, but it rarely works the other way round. Managing expectations is a fundamental rule in business. No-one gets it perfect, but those who get the "balance" right are the ones that survive.