I know that I feel uncomfortable in clothes that aren't my style. So make sure that you are wearing something that is you. I wouldn't change your hair dramatically right before pictures, especially if you are not used to it. In my experience you don't feel or look like yourself. The more outfits you have the merrier.
A big rule of thumb on finding outfits is to make sure you DO NOT MATCH! Coordinating is the better way to go! I usually like to find moms or the youngest daughters outfit first. Trying to make sure there is a few colors in it and then use those colors on everyone else. Try to avoid a lot of patterns or wording, these can be very distracting. Even in summer it is better to have long sleeves and long pants. The less skin showing the more that can be hidden.
Stay Patient! Newborns take a long time! They want to eat and then their diapers changed and then they want to be clothed and then. . . we are on their schedules. The best time is within the first two weeks. It is ideal for the new baby look. However, if you don't make it within that time period, make sure it gets done when you can.
Please bring any blankets, decor and or props from where your going to be putting these portraits, that way they tie into your home. Mommy and daddy pictures are fun keepsakes. Make sure you are prepared yourself for being in front of the camera!
To help me get those real smiles make sure to keep a calm attitude and give very positive feedback. I am in no hurry! Kids will come out of their shells when they are ready, and I will be ready when they do. It is always kind of fun if they have a spunky fashion sense to let them pick out an outfit to wear. (You never know it might help them feel more relaxed) If they have a favorite thing to do or play with BRING IT! It is all about THEM and seeing this age.
Your personality needs to mesh well with your photographer. No matter how much you LOVE their work, if you can't work with each other, you will not love YOUR pictures. This is one of the most important times in your lives and you have earned the right to LOVE your pictures. I promise from personal experience THESE are YOUR memories! The day goes by too fast to record everything to memory. If you are not going for a traditional wedding and are okay with letting your husband-to-be see you in your wedding dress. I would highly recommend pre-shooting your Formal portraits. It will make that day go much more smoothly.
If not, then you should plan 2-3 hours for these. If you are wanting formal family pictures then you need to plan about another hour before the reception.