1) I use different hosting (therefore different IP's and DNS) for each Blog
2) Private registration of domain
3) Different theme (can use the same but not all the time)
4) Don't put Google Analytics on them or register with Webmaster tools
5) Don't put adsense on them (huge footprint)
Different hosting may sound expensive but you can use the account for more than one Blog (niche) as long as you don't ever link them together and they are in different niches. I only ever link the Blogs to my main money site to pass authority and never link the network together anyway.
You could use a VPS / Private server but then you need to have different Class C IP addresses and DNS. I find it easier not to have to worry about it and different Hosts solves the problem as you merge in with the rest of the herd on that shared server.
Be aware that when linking the Blogs together if you do it incorrectly the Blog network will be next to useless. Also consider buying aged domains instead of building your own.
