f you've been in business for a while, you should have some record of what sold and what didn't sell throughout the year. Analyze that data. Break the inventory down into categories and try to correlate the categories with customers. Trends or patterns will likely emerge. Depending on what business you're in, these might correspond to the rhythm of the seasons, or they could relate to the financial year-ends of your biggest customers.
Once you've discerned a trend or pattern, you can begin to plan acquiring and paying for the merchandise you require according to your needs. The idea is to stock the inventory just before you need to sell it, so that the money you've invested comes back into the business as quickly as possible.
A good inventory information system should allow you to capture sales history, and forecast sales to come, based on that history.