Unlike food, good music doesn't have an expiration date. This is a very good thing to remember. I see so many today who promote just one song or one mix. Usually their newest release. And that is good, but you need to promote all of your good tunes, not just the newest one.
Just because a song is old to you, it may be new to someone who has never heard it before. I have songs that are ten years old I still promote to this day. I continue to see sales on tunes from 10 years ago. Someone just bought my first public album release last week. That album is over ten years old.
If you have a catalog, you should be pushing all of your great material. You never know which song will grab someone. I have had people write me and tell me it was a song they just ...full