Examine all aspects of the yard to be landscaped. Will you need to level or hill up any areas? Are the angles good as they are or do they need a little help? These kinds of things matter and will have some part of the drainage aspect mentioned above when it comes to your landscaping plan.
Most people do not have a million or so dollars to spend so you will have to take this into consideration when you are coming up with your landscaping plan. You need to know how much you will be able to afford before you go all out designing this amazing, yet expensive landscaping plan. You will probably not be able to afford to change the whole slope of your yard even if you wanted to so try to stay within the scope of your money and your ideas.
Make sure that your landscaping plan has taken everything into consideration and the and only then should you start putting this plan into motion.