How to use HF Exciter in the mastering process to add "shine"?

A lot of the commercial sound you're hearing is probably either use of an Aphex Aural Exciter or a BBE Sonic Maximizer. The trick with HF Excitement is that it can keep sounding better and better as you push the highs louder in volume, only to realize 30 minutes later that your ears are seriously fatigued and you need to take a break. The BBE unit was famous for this problem, which is ironic since the phase alignment process at its core has nothing to do with the Lo Contour and Process knobs that bump up 50 Hz and 10 kHz respectively, and were the source of a lot of "overdoing it." I've found the best results when keeping the BBE knobs below 1 (they go up to 10!) Aural Exciters can be great tools, but please exercise restraint. :-) Peace, Tunes

Again, I know that every mix is unique and different, and it's always best to trust your ears and do what sounds best,

That really is the best way to do it. And never trust your judgement on a single session.
 
Go back to the track sometime later (hopefully before release) and make sure you've not overdone it. A catch with exciters is that the longer you've been listening to them the more the brain kind of gets used to hearing the effect and it sort of disappears, so to maintain the initial effect you need to add more and more excitement........

but I wanted to see if there was a general outline for using exciters because I've never used them before.

As tunekicker says - "use with restraint". Remember that things like iTunes contain their own exciters, which are generally on by default, which will be added their own boost and fizz to any exciting you do and while too much exciting will grab a listener's attention it quickly becomes very fatiguing and uncomfortable to listen to.
 
So if you write the kind of music that you want people to sit and listen to or as comfortable background ambience you might not want the same settings as someone recording a track aimed at the charts or clubs where it really does need to stand out and instantly grab ears.
 
It's also worth remembering that a huge amount of very good music was very well recorded and mastered before anyone thought of exciters....