Clean your leather with the grain and not against it. Should the zip on you leather bags become difficult to open and close, a little beeswax should do the trick, although zips get smoother with time.
If your leather bag gets wet(water), do not use heat to dry it will completely change its structure, stretch out the bag and change its shape. Let it dry naturally.
If it stains/leaves a mark: Immediately when it happens, take a damp cloth and gently wipe the area. Do not apply too much pressure. Stains and oily marks are nearly impossible to completely remove, so please try handling all your leather with clean hands.
Store your leather bags in a dry & dust free space, away from heat or moisture
Overfilling your bag will with time stretch the leather out, keep it light