[FONT=courier new]Guess it depends on what exactly you want to do. If you are only ever going to 1 related product for each product then you could include a 'related' field in your products table which includes the productID of said related product.
If there are likely to be differing amounts (i.e. some have no related products, some have 1, some have 5) then you may want to look at having a lookup table which stores the 2 productIDs in each row.[/FONT]