Well, I just inherited a '97 Deville that had a seriously neglected leather interior and this is how I handled it:
1) Wipe it all down with a damp (not soaking wet) cloth to pick up all the loose dirt and dust.
2) Apply a good quality Saddle Soap, making sure you get into all the leather's nooks and crannies. I used a toothbrush all around the stitching to ensure nothing was missed in this important 2nd step.
3) Wipe down the leather, removing any residue that the saddle soap may have left. Allow the leather to "rest" until you are sure it has dried.
4) Apply a good quality Mink Oil to all the surfaces in a circular motion to ensure 100% coverage. This stuff will re-introduce any lost oils and nutrients to the leather while waterproofing it at the same time. Allow at least 20-30 minutes for the treatment to soak into the leather.
5) With a soft, lint-free cloth, begin rubbing out the leather, turning often to get a fresh part of the cloth. Make sure you address any creases or natural folds of the seating, as you don't want to risk getting any of the Mink Oil on your good suit (or your lady's new frock. Believe me, your life won't be worth living if this happens).
6) Finish with a good, brisk polishing of the leather with a fresh cloth and Voila! A new interior you can once again be proud of. Smells like new leather, too.
Hope this helps,,,,,,,,,,,,,,,,,,,,