are you bi-lingual? Speaking to them is the best way. There are books and games that are also spanish and lots of dvd's. I have seen a few things at Borders in the kids section. We speak to my daughter in both languages she is 7 month old but they say to do it so they can hear them and pick both up. we want her to be tri lingual but we will introduce the 3rd language around 5 or 6 years.
Good luck there are a lot of neat things out there you can use. I like the books and dvd's. I have a friend that would put in their favorite movie and change the language to spanish and let them watch it that way.