Keep your bag stuffed while you're not using it... to help keep its shape better.
Put your handbag in a dust bag or a pillowcase.
Apply a protection cream regularly. Proper protection can prevent stains, grease penetration, and keeps your leather bag in perfect condition.
Use a professional leather cleaning agent on tougher dirt. Never use baby wipes, vinegar or any other 'home remedy'.
For really stubborn stains... try matching a shoe polish to the colour of your bag and buffing a small amount over the affected area.
Wipe with the grain of the handbag... to avoid ruining the leather
Always spot-test a cleaning method before you use it.
When cleaning always use a product designed for use on the material. A general leather cleaner will not be as effective on a suede or nubuck interior than a cleaner specifically designed for suede & nubuck.
Avoid holding your handbag if you've just applied hand cream.
Don't leave your handbag in direct sunlight.
Last tip I found, but didn't try it out yet : if you would have an odeur in the bag, because of something that was leaking, to banish ever-lingering smells... put an open container of baking powder carefully inside your bag, place your bag into a dust bag or pillow case and leave for 24 hours. The baking powder should absorb any unwanted odours.