Make sure that garbage is cleaned out from your vehicle each day. Don't leave food or other perishable items laying around. They will spoil and smell.
If you take your pets along with you, put a special blanket or cover down to protect your interior. You can wash the cover when you get home.
Periodically, it's a good idea to air out your vehicle. Leave the windows open for a few minutes. You can use an air freshener product to deodorize if need be.