Linking To My Other Domains

I would appreciate if any of you can offer me some suggestions to improve my page rank, seo, etc.

I have my main website for selling one product. Just got it going.

I have also purchased 4 or 5 additional domains that I may also use for this same product to sale

How would I link these other sites to my main site and back again. Do I do this thur the link section on my sites or is there a different way to do this...

Also, are there any other tactics, either white or black hat techniquest that I could use but still protect my main site so that it will not be banned from google. etc.

Thanks
B.