At the risk of starting another 'food fight' like we had with Dawn:

Verts pose special problems (UV, tree pollen, dust, precip and sweat.)
  Stick with the suggestions in the owner's manual.  Damp cloth, maybe
with mild soap or saddle soap, no detergents and sparing use of the GM
solvent based cleaner if you get a grease smear on the leather.  I'd add
vacuuming and a damp wipe down each time the car is washed.

Over the years, I've had poor results from different commercial leather
cleaners/dressings.  I think most have contained silicones.  In the
short term these provide a quick "look-good."  Over the long-term, I
think they dry the leather and cause cracking.  My C4 ('89...61K miles)
is showing that type of cracking and I believe that the prior owner was
a leather dressing nut.

My '02 is getting the owner's manual treatment -- at 5+ years of age the
leather is still supple and shows some burnishing from my wearing
cotton-poly trou.  My wife's cars are traded more frequently but since
1996 I've stayed with the owner's manual suggestions. Before each 6 mos.
detailing I warn the kids to not dress the seats.

My neighbor's son is using Neatsfoot saddle dressing on his '04 seats.
Probably OK for the leather but the vinyl no longer matches the leather.

--
PJ

I used to use saddle soap occasionally to clean them and then a good leather
conditioner. A friend owns a shoe store and I'd get the stuff from him.

However, a couple of years ago, I was getting some leather seats from Al
Knoch and he said the saddle soap will dry the seat covers out too much, and
the conditioner would promote cracking.  He said his recommendation was to
simply spray with Pledge and wipe clean.