IE8 CSS Hack - best method?

As other answers posted,you could use Conditional Comments,but that would add one HTTP request when user is using IE8. You can use the \0 hack in the css file

img{
width:200px;//other broswers
width:100px\0;//only IE8
}

You should reverse your method. First your site should look good in modern browsers (like Firefox, Chrome, Opera, IE 9), and then you can start worrying about the others.

As others suggested, conditional comments can be your friend.

First, you should develop your CSS to look fine in the modern browsers. Then check IE8, see what problems you get. If you need to, include an IE-specific stylesheet. After that, you can check in IE7 and then IE6 if you support it, and add further fixes.

An example:

<link rel="stylesheet" href="normal.css" type="text/css" />
<!--[if lt IE 9]><link rel="stylesheet" type="text/css" href="ie8.css"><![endif]-->
<!--[if lt IE 8]><link rel="stylesheet" type="text/css" href="ie7.css"><![endif]-->

In this case you include normal.css which is for modern browsers. You found some strange IE8 issues, so in ie8.css you fix the problems. You don't have to include all your selectors in this, only the ones that need a fix (the values will be overridden for IE 8 and lower). After that, if there are still some strange things in IE7, you can add your ie7.css and fix those, and so on.

Please refer to the links the others gave you to get more information on the usage of conditional comments.

Finally: making IE8 render as IE7 for ease is never a good idea, and should be avoided. IE7 is the far past (in the IT world, IE8 should be the far past either...), develop for the present and the future, and after that you can care about the people who are still stuck with old technology (based on your audience and business plan).

Use conditional comments.