Background Information:  I purchased a Home that is approximately
10yrs old.  The lawn is stable.  As far as I know there has been no
treatment of the lawn other than the occasional watering.  I have
researched Dethatching and Aerating to improve a lawn that has
stabilized.  I have no idea what type of grass/weed mixture is present
in the lawn.  I have noticed a very thick stemed weed with a "wheat"
looking head that grows in predictable locations of the lawn.  These
weeds trouble my lawn mower and usually take a couple passes to cut
down.  This happens between mowings of about 1.5 weeks.

My goal:  I would like to create a lawn that is more than just a
combination of random types of grass and weeds that just "happen" to
grow there.  I would like to introduce a prominent grass to my lawn. 
Think of it like getting a cover up tattoo.  I want antother tattoo,
just better than the one I have.

My question:  What are the steps to creating a lawn ontop of a
sub-standard lawn that already exists?

When I lived in the Knoxville area, I had a neighbor whose weedy,
patchy lawn was given a remarkable makeover with the help of Amazoy
zoysia plugs.

http://www1.zoysiafarms.com

Really?!  'cause I live in the Knoxville area, and I have exactly the
same problem as SgtPoe.  Those stupid weeds shoot up practically
overnight, and then they "duck" when the mower goes over 'em.  I swear
I can hear them laughing...

I can't promise that zoysia will be the answer for everybody, but my
neighbor's lawn looked better than any lawn on the block, and he
credited the Amazoy plugs for having crowded out the weeds. I'd have
given this a try, but I was living in a rental house, and didn't want
to invest much in lawn maintenance for a house that wasn't really
mine. This was in Hall's Crossroads, north of Knoxville, if location
makes a difference.