the biggest thing is having all your title tags meta tags and links matching exactly the way they were before if you want your rankings to hold
if your site wasn't well optimized to begin with wordpress is going to help you out a ton because it is so search engine friendly in design
i always recommend downloading the 'All in One
SEO pack' from the free plugins.
it will help your site's
seo structure once its installed at the bottom of the pages when you are editng them you will see the all in one
seo pack fields that you want to fill out evrytime you create a new page or post on your site
it's well worth the extra minute or 2 it takes to fill out that info every time you create a new page.
by the way using redirects is not the
seo friendly way to preserve your rankings you want all your pages to have the same links they always did. which is time consuming but worth it if you have some good organic traffic coming in