Waterproofing is a great way to protect your bag from rain, snow, food or drink. Most drugstores and shoe repair shops will carry a leather waterproofing spray. Make sure you choose a product for leather, and not for fabric or suede, as these products could ruin your bag. Also, read the directions thoroughly, as spraying too closely can cause stains or discoloration. You can also have your bag waterproofed professionally at a leather or shoe repair shop.
To clean your bag, you can use the old fashioned "Saddle Soap " that is also available in most drug stores. Be careful to follow the instructions on the label as getting the sponge too wet, or using too much product, could damage your bag. Saddle soap is great for removing regular dirt and grime. Other gentle leather cleaners and conditioners like leather lotion, can be useful as well. If you have a tough stain, you might want to take it to a professional, such as a shoe and handbag repair shop.
Store your handbag in the cloth "dust bag" it comes in when you're not using it. It's a great protector!