Can robot.txt help in Domain Move?

[Rocknroll]

Hi all,
I was just thinking about moving my site to new domain name. And to avoid loosing rankings, I was thinking about doing this.

1. Copying the content to the new domain (still keeping it on OLD!)

2. Putting robot.txt on old domain to stop indexing the OLD site so that I am not duplicating the content for SE's

3. And eventually when my NEW domain start to show up on the SERPs, I can place 301 redirect from OLD to NEW domain!

Now here is my issue. I believe robot.txt wont clear the files SE's already have from my site in their Databases. SO when they crawl my new site, will they still penalize me for content duplication?

OR my ONLY choice is to put 301s right away! Any other ideas are welcome!

Thanks