If your question is how can you plan for a specific gender, there are a bunch of old wives' tales. However, I have read that a high/low ph in your body helps one or the other. I don't remember which. You can probably find out that info on an online site about pregnancy, etc.
If you are planning for clothes, toys, etc. northweststarr's answer works. If you have several children, you will just dress them in whatever is clean and handy, when they are very small. You will have to change them many times a day, so it is not worth worrying about.