i find mastering the most fun aspect of making music. but it really is bringing all your experience into play to make that crap track sound good. i'll sit for hours mastering tracks and the results will get worse and worse, and worse. i find the best way to master is to go into the studio with a totally fresh head. you haven't heard the track for months and you bang it on and it's like - WOW that's so obvious. mid eq goes up, hi's go down, limiter goes on, compressor kicks in bang bang bang and you're like 'holy ***!!'.
the key is quickness and the only way you'll be quick is if you do it like this in my book. it's like putting everything you've learnt into play in a 100m race ;)