How do you shop for 6 months of groceries & supplies?

One of the common questions that we get asked is where do you shop or even how do you shop for groceries? The answer is that we start now in August to plan what we need to the winter in the Bahamas. We buy as much dry goods as we can here and store it in our pantry. In the next 6 weeks almost everything that we need to buy will go on sale, at some point in time. We use various methods to plan purchases:

1.) We inventory everything on the boat from batteries to vitamins and calculate as best we can what we will use over the winter.
2.) In previous years we have tracked our purchases in Canada, in the US and in the Bahamas as well as our actually usage. We use this information to plan for what we need to buy.
3.) Some things are cheaper in the US or even in the Bahamas (rum) so we buy certain things along the way.

Sometimes its hit and miss and other times we have it figured out based on previous winters. Its really more of an art than an actual science.