Four of my 10 grandkids live across the country from me, in California. Like you, I visit once or twice a year, and have been since the first one was born, and she's now 15. It's hard maintaining a relationship with them across that distance, especially when they're younger. It's easier now, because the older two have their own phones and so we text and send pics all the time. I do the same thing with the grandkids here who are old enough for phones, Facebook accounts etc. I use any medium I can to stay connected.
Ever since my California grandkids were born I make a point to send them each, separately, mail, cards, letters all the time. Every seasonal event they each get a card...Halloween, Easter, Valentines Day, Thanksgiving, Christmas, St. Patty's day, any excuse for a card, or sometimes no excuse at all, just "thinking of you" type cards...plus cards on their birthdays of course. I've always tried to include something in the cards..stickers, temporary tattoos, glitter, cutouts..whatever might be appropriate for the day and the age. On their birthdays they get a little money in their cards.
But too, their parents/my son and daughter in law, are SO good about keeping me "alive" in their kids' lives. They refer to me often, tell them to calls Grams about this or that, refer to me as though I were just down the street instead of across the country. Too, they send me stuff about the kids all the time...information about what they're involved in at school, their latest recital, that Katey hurt her leg and had to wear a boot, a funny pic of Chari who had dressed herself and OMG!! Stuff like that, which gives me additional information that I can talk to the kids about, and refer to, and make them feel like I know what's going on in their lives.
I love my grandkids and I really enjoy the relationship I have with them. That said, sometimes it takes work to maintain that relationship because often it's a bit more one-sided. They're young, and more self-focused at that age, lacking the maturity to maintain meaningful relationships outside their immediate sphere. So it's more work on MY part to keep up the conversation and interaction. Very worth it though!!!