What to do after doing 301 on old site?

Hi looking for a bit of advice on a few points... Building a new site for a client on a new domain... Once finished going to 301 the old site`s individual relevant pages to the new site. The old site is using Blogger with custom domain...so would you redirect within Blogger or the htaccess file? Also am I right in saying the link buildout should still use the old domain URL as the new domain will be sandboxed for a few months? If that`s the way to go am I also right in saying I can be a little bit more aggressive with the on-page SEO on the old site as its Google`s baby? Any input would be great..