When working with ancestors (or any other spirit), it is important to remember that our relationship is a reciprocal one. The spirit world is not a place where entities wait patiently to dispense free enlightenment, prosperity and better sex to all who ask. Remember the old Hermetic axiom: as above, so below. You expect to be compensated for your time and efforts, and so do the spirits. By making offerings to the ancestors you will strengthen the connection between the worlds: instead of relying on miracles out of nowhere, you set up a system of exchange which provides continuing benefits to both sides.
As Nutty Professor pointed out in the comments to my earlier post, food is an excellent offering. If your great-grandfather loved chocolate cake in life, why not give him a slice now that he's shuffled off this mortal coil? This allows him to relive one of his favorite sensory experiences. African tradition says the food of the dead is to be prepared without salt: if you are making food specifically for the ancestors you might want to keep that in mind. However, I have found that you generally can feed them food from your own table, even if salt was used in the preparation. This is especially true if you are cooking a traditional family meal or something which your ancestors particularly enjoyed.
There are good practical reasons why the Holy Eucharist began as a communal supper: bread and wine sealed a covenant among fellows and provided spiritual as well as physical sustenance. Leaving a plate for the ancestors along with your own food helps to connect you and your family to those who have gone before you. It also sanctifies the act of eating, turning your meal into a prayer. Your food becomes not only nourishment for you but for your lineage... and people, like plants, grow best when their roots are well-fed.
That being said, avoid consuming the food which you place out for the ancestors: do not nibble from their plates or drink from their glasses. The dead consume the essence of what they are given. When you eat their offerings, you are exposing yourself to spiritually tainted food. (You may also be exposing yourself to bacterial contagion, as I have found offerings to the dead will frequently decay at a greatly accelerated rate). It is important to establish communication between the realms of the living and dead: it is equally important to recognize and honor their boundaries.
This devotion can be practiced easily and discreetly. One of the most brilliant and dedicated Houngans I know leaves a bite or two on his plate at every meal. When he is done, he taps discreetly on the table, thereby alerting his ancestors that what remains is theirs. Thus he feeds his ancestors and commingles his spiritual and material lives without attracting undue attention. Learning to pray quietly and unobtrusively will increase the opportunities in which the ancestors may involve themselves in your life. It will also serve you well when you begin to explore more challenging and potentially dangerous paths of spirit-work.