One of the big questions that you have to answer, especially if you are talking about a holiday picture, is what do you want to have on your wall?  Or are these images just for a card?  For example, Santa hats and reindeer antlers may look really, really cute on your card but probably not as cute as the main 2008 image in your family gallery.  This can even be true with Christmas inspired dresses or sweaters.  There's no right answer except what your taste dictates.  If you want to have a holiday feel but don't want the image to be strictly a holiday picture, you might just do it with fall colors or holiday colors.  You can also do a few shots with the cute hat or sweater vest and then take it off for something less seasonal.

If the images are destined for the front of a holiday card, keep in mind that you can have the card background reflect the holidays and keep the clothing more universal.  For example, like the pictures above and below, use a holiday inspired color in your clothing choice and then finish it off with a great card design that reinforces the holiday feel.  The image below would look great on a holiday card but would also look timeless in someone's family gallery.

When it comes to family shots, it gets a little more complicated because you have more people to dress.  Here I think the first question is whether or not you are a formal or casual family and which of those looks you want to put forth.  Here the family dressed up and gave a nod to their heritage by choosing the kilts.  The other thing to note here is that, although they do not all match, they are coordinated.  There is a theme and it makes it so much easier on your eye.

If you consider yourself more casual, then the next image may be more your style.

With new babies I think it helps focus the attention onto the baby if the parents wear simple and uncomplicated clothing.

When photographing a newborn, keep in mind how small they are.  It's hard to find clothes that fit when they are brand new and often the clothes overwhelm the baby.  Watch out for collars that will stick up and block their faces.  For example, button down shirts with collars are terrible on babies who have no head control because the collar covers them up.  Personally, I think brand new babies look best in solids or clothes with very little pattern.  For the most part, they also look better with only a diaper cover, swaddled in fabric or with nothing at all.  Hey, when else can you parade around naked and everyone just looks on thinking, "oh how cute!"?

When you do pick out outfits for your kids, always have them fit!  As parents, we always buy a little big so they can wear it longer but big in a photo just looks weird.  It will make them look too skinny and small if they are a newborn and just slouchy as they get older. 

If fashion is part of who you are as people and as a family then you may want to go a different route.  If you love bright colors and funky clothes then that is probably what you should have in your images.  In this case, you still need to coordinate but you can be more bold in your choices.  Always remember, this is a picture of you and your family.  It should reflect who you really are.

The main thing I want to say is that you do have to think about clothing choices.  It will make a huge difference in the pictures that you are taking and the ones that the pros take as well.  Think about keeping the images simple, think about colors and plan what you will wear and you'll be much happier with your results.