The best way is to avoid expressing them as much as possible in front of the child. If you can suppress your reaction, you will minimize the impact of your fears on your child. This, however, is often easier said than done and if you have a more severe response or are startled by something the best thing you can do is explain to your child that you're okay and that everything 'is fine'. If you can reassure the child and just explain that you were startled or surprised or that it scared you but that's all, that there was nothing really to be scared of, you can help him or her understand that it wasn't a big deal. Phobias are essentially irrational fears and by explaining that in an age appropriate manner, you can reduce the chance of passing them on to your children.
For example, saying things like "it's okay, there's nothing to be afraid of, Mommy/Daddy was startled by the spider" or "snakes are something that bother me but there's nothing really to be afraid of because there aren't any running loose around here" can help your child understand your reaction. With small children, it's important to keep your explanations simple with statements like "I don't like heights but it's okay for you to go up on the slide". Reinforcing that you might have a problem with the subject of your phobias but that it's okay for your child to be near it/them (like in the heights example above).