The following blog-post from Darcy Pattison may answer your questions:
http://www.darcypattison.com/pr-notes/online-contest/
In it she interviews 3 authors who have blogs, at least 2 of whom write for children, about their experiences with running online book promotions. The main tips are: keep it simple to enter, put 'Contest' in the title of the blog-posts that talk about the online competition, and keep the length of the competition short.