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.
Corrective Plan. A marketing plan is a living document. Trust me, the moment you have it and experience planning your marketing, you will realize this. Therefore it is a plan that allows you to assess past strategies and results and at the same time allow you to make necessary corrective measures to fit your business growth. for example, who expected the 2008 financial crisis to hit us so badly? No one other than the few so called "economists" but how did responded to it? Severely. Many businesses closed including many giants. But the good thing is, many have a solid marketing and business plans, therefore they make quick corrective actions and moved forward.