I married 12 years ago, and from the day I met them, I got along well with my husband's folks. The day of our marriage, they took me in as their own daughter, and encouraged a good relationship between the four of us - maybe it's because with 11 children (a wonderful Mormon family!), they had had a lot of practice!
There has been a great sense of balance in allowing space but maintaining closeness with us. They involve us in decisions, and ask questions about future plans, such as "Would you like us to visit you guys this year? What time of the year would be best?"
They are totally flexible about celebrating a holiday on a different day if we're trying to 'share families'. However, I feel it's important to make sure that we share time evenly between the two sets of grandparents, and we trade off spending the actual holiday with each one, every other year, or by trading off Thanksgiving and Christmas each year.
Beyond just remembering birthdays, they also call just to talk with me for no particular reason sometimes. I love those talks, even though they're usually brief. The conversations always end in an expression of their love for me; and me expressing my gratitude to them for raising such a wonderful son and for teaching my husband how to be such a great Dad to our 4 children.
With this kind of frequent and positive communication going on, and with all of us being flexible and open-minded, it's hard to find any reason for bad feelings between us. Sadly, my husband lost his mom to cancer - but his amazing dad has kept the traditions alive, continuing to call often, and send out birthday cards to all 35 of his grandkids!! Wow. What a blessing good parents are to their posterity. I have learned so, so much from them.