Graceful degradation / Progressive Enhancement best practices

What are some best practices in handling graceful degradation/ progression enhancements while developing a website. For example, is it better to use conditional html tags to load some javascript:

<--[if IE]>
   <script>ieScript.js</script>
<[endif]-->

or is it better to use try catches?

try{
   nonIeCompatibleCode();
} catch(){
   ieCompatibleCode();
}