The fact that you're asking this question means you probably need to look at starting smaller...
Build networks in blocks (we do them in blocks of 250) and spread them across at least 50 C classes and 5 A/B classes. This means no more than 5 sites per IP, and avoid using similar themed sites on the same IP. Don't interlink in any way and avoid any kind of situation that could indicate shared ownership. This does present a problem if you're using AdSense - as it's an automatic pointer to a single point of ownership.
Use reliable C Class hosting providers. There are providers that have over 200 IP classes available, but I would put each network of 250 under a different hosting company and registrar etc. basically, further distancing them.
You also need to consider why you're building them. If they are to be a pure linking network then your build strategy will be different than if you're building a niche site network to hit whole groups of keywords etc.
Thanks for all the info jascoken. I really appreciate itA few questions tho:
1. You don't worry about having multiple sites on the same IP, even if it's only 5 sites? Won't G catch on to a pattern if you set it up like this? Or do you mean same C-class but 5 different IP's?
2. What are some of the reliable C-class hosting providers you speak of?
3. What's the main difference in build strategy between a pure link network and a niche site network? Just the fact that a pure link network won't interlink but a nice site network will?
4. Have you ever considered setting up a network with many different cheap hosts and/or free hosts, rather than a c-class hosting provider? Or would this just be too hard to manage?
5. What software do you use to manage your network (add posts, etc)?
Sorry for all the questions. Happy Holidays!