I've learned these tips for designing your own Christmas card:
Gather inspiration. Look at other card designs. It's fun to see what pro designers have put out there. You can get a lot of great ideas from these sites: http://www.minted.com/ and http://www.tinyprints.com/.
Have your family pictures taken in the fall. (We've waited until November before, but it is tough to squeeze them in. Our general rule is to get pictures done around the first couple weeks of October.) This year, I asked a fellow photographer to trade shoots with me. I shot pictures of her family with her camera equipment, and vice versa. Since we both knew what we were doing as far as posing, clothing coordination, and settings, we each got great images to take home and edit ourselves. (Big plus for me!)
If you decide to go the tripod and timer route, don't expect more than one or two photos to turn out. Self-timer pictures can be done, but they take a long time, and kids tend to fade quickly with the tripod. From my experience, it is totally worth it to get a photographer to take your pictures.
Pick one to two of your favorite images to put on the card. I've seen cards with so many pictures, I can't tell which is the most important. A good rule of thumb is to keep one image dominant by making sure it is at least 3 times bigger than the other elements on the card. Of course, this rule may be broken if you are doing a series of pictures of the same size -- often in odd numbers. In that case, the set of images itself acts as the dominant element.
Keep text and fonts to a minimum. I really think the photo (or photos) should be the most important part of the card--after all, that's why you are sending it--people want to see you. As for fonts, I rarely use more than two font types in a design. If I'm using a fancy font for one piece of text, I'll use a plain one for the other. Good design needs contrast. If you have too many elements competing with each other, then the design is weak.
Get help designing if you need it. I design my card in Photoshop. But you don't have to have a professional design program to make a great card. If you are having your Christmas cards printed online, see if your vendor has an online design program. If you want to do something totally unique, contact me for custom designs or get help with an online/phone mentoring session.
Proofread! It's embarrassing to have misspellings or grammar mistakes in your card. Make sure to print up a sample and proofread. Better yet, have someone else look over the card. I hate to admit it, I've seen a few mistakes on Christmas cards I've received this year. Whoops!
Have the cards printed at a reputable printer. We print our cards at http://www.overnightprints.com/. They offer great prices and are professional printers that color manage. If you get cards at a discount printer that does not color manage or use professional inks and paper, you'll probably be disappointed. It's not fun sending out a ton of cards that don't look right.
Save one or two for yourself! Your yearly Christmas card is a great piece of family history that you'll want to look back on every once and a while. I keep our old Christmas cards with our decorations so I can look at them every year. It's fun to see how our family has changed.
Merry Christmas and a Happy New Year!