I have been involved with promotions for brands such as: Energizer, Toyota, Osh Kosh B'Gosh, LeapFrog, Microsoft Hohm, Wendy's and many, many more.
In the beginning, I never had success randomly emailing companies. If you are too intimidated by that concept, start off joining communities like: Mom Bloggers Club, BlogHer, Mom Select, Mom Central, The Motherhood, etc. They already have relationships with brands and can match bloggers to opportunities.
Create a media kit. Companies want to know what reach you have and that you will represent them well.
Once you have a little experience with sponsored posts, I recommend attending blog conferences, such as: BlogHer, Blissdom, Type-A Mom and/or Mom 2.0. The brands are there. You will meet some face to face. This builds trust and you will both feel more comfortable establishing a relationship going forward.
Good luck :)