II-V-Is and their uses

The basis of most of my reharmonisations can be found in the well known ii-V-I sequence.  I've explained this in a previous post.

One thing I've started to notice as I've began to develop and reharmonise existing tunes for my portfolio is that I'm using the same rules over and over:  the rules that extend vanilla chords into deeper chords (eg. a A major triad into a Cmaj7 or an A minor triad into an A minor 7).  I'm then using other rules to turn those extended chords into ii-V-Is, then simply reharmonising the ii-V-Is using tritone substitution or other rules.  By using this simple method (and adjusting to fit the melody), I can make most contemporary tunes sound reasonably smooth-jazzy.

I'm using rules on rules on rules.  By doing this I'm ignoring a large number of the rules I've created, but it seems to work a lot of the time.  Perhaps this is me unintentionally whittling away at the draught ruleset I've created.  As the weeks go by I'll post smaller and smaller rulesets until there's only a few core rules remaining.

Using and abusing ii-V-Is like this may seem like a cheap method, but for what I'm trying to accomplish, it seems to be working wonders.  There are still songs like jingle bells and amazing grace that I'm struggling to apply these rules to.  I'm not sure why, but those songs seem to require a little more work.  Rules that work for many other tunes simply do not work on these ones.

I'll try to investigate why this is and post my findings later.  

J