Which one is better to use for duplicate content issues, canonical Tags or 301 redirection ???

Canonical Tag would work fine if you need to have the Page in place for various reasons.

If i am getting link from Forum and if they got High PR, than should i go for these kind of links??

yes, mix of links from good sites are always good

Sometimes we do changes in on page, like changing Title,meta,des to do experiment.Is their any way through which i could know that changes are made for good?

There is no online tool to check whether the title & meta tags are good for website or not. Regular monitoring of rankings & traffic will help you to understand the result... You can track the ranking for different keywords and see if there is a positive push.

What should be the ideal title length of any "Anchor Text"?

It is the keyword you target to, depending upon your business. ideal =<4