I have my personal website running on my main domain like myname.net
I want to buy some domains that have great keywords to me, like: professionalrj.net and freelancebrasil.net (just examples).
I don't want to redirect these new domains using 301 since I want to "score" with the keywords relevancy.
I know that I can use canonical meta tags to tell google what's my main domain, but I don't know if it's enough the prevent duplicated content problems.
Watch this video by Google about Canonical meta tags:
This page also contains Q&A that should elucidate your doubts.
Great link in Google Webmaster Central blog with a presentation:
Canonical Link Element: presentation from SMX West
Hooray Brazil! :D
I would strongly advice not to go this way.
Using the same site to boost up keyword relevancy is not a good idea as you are creating duplicate content. Solving the issue with a "rel=canonical" is not a good idea because the canonical tag is only cross-domain for Google. Additionally you are telling Google "don't look at this site". Investing in domain names just for keyword relevance is probably lost money.
To boost up keyword relevance, get (good) external links!