I recently have started using an awesome little JavaScript code to help me get over the time wasted developing my CSS and making it compliant with IE6. The code takes the HTML and CSS and makes IE6 display it as IE7 would display it. They also have some in the works for IE8 as soon as it comes out. This code helps to allow you to only develop for one version of Internet Explorer. It has been a life saver. I haven't run into any problems yet. So far so good.
When ever I design templates I always try to ensure that all of my HTML elements are styled properly. It's very easy to forget element and to see how they look. I've tried to create a test file with all the code you will need to be concerned about on a basic level. You will find that every template and every CMS may introduce new specific styles you need to be aware of, but from a basic perspective this should cover them all. Let me know if you find more that need to be added and I'll make the proper revisions. I am far from perfect and I'm sure could improve my process.