I have a website that has pretty decent SERPs and I've been working on the SEO for a while. www.domain1.com There is a forum on the site: www.domain1.com/forum I want to get a new domain just for the forum. So then it will be: www.domain1.com - the main site www.domain2.com - the forum So it's not changing the whole site's domain, just the domain of the forum directory. Is there any way of doing this and keeping the forum's SERP's around the same? Is there a smooth way of doing it?
No matter what you do, there'll be some change in the traffic and rankings for the forum till the time Google updates its database with the new domain. Also, the forum now is a part of your website. It is imperative that your website is gaining some benefit/power from the forum. One thing that'll reduce is the number of pages indexed by Google for your site will eventually start reducing due to this change. So, you have to be prepared for all of this as these things may happen and reduce your income if you have monetized the site and the forum.
I've heard stories that somebody has redirected via 301 and took care of everything, but the SERPs have still dropped..
Yes 301 redirection is the best. It may take time to have perfect effect of it in Google. You need to wait for your new domain to get same popularity of visitors. Also you will loose backlinks so you need to start from scratch.
Everyone tells you to redirect. And that is what you should be doing. To add to those points, just make sure that you do not build links fast to the new site. Fresh domains take some time to establish trustworthiness and so you need to wait a bit before promoting it further