You need to know everything obtainable about your customers. This list will get you started. You will need their age, sex, marital status, income levels, education levels, geographic locations, where they shop, if they go to church, if they have kids, ages of children, political affiliation, what they like about your company, what they don't like, why they shop with you, what they value about your products/services, what they like the most/least about products/services, and whether prospects are unaware of your brand or products/services. Customers want you to know certain things about them and if you do, they will remain loyal to you as you take time to know them.
In the end, we should not forget to specify how the plan (or plans) will be controlled, by what means we will measure its results.We will see how to build the marketing plan, what is its structure: after we will see how to build the traditional marketing plan, we will take a look at the e_marketing plan and see how the unique features of the internet will require some changes in the approach of writing a marketing plan.