What are effective optimization techniques for Views 3?
You first should find out where the performance bottleneck is.


Is it the executing of the query
Is it the rendering of the view
Is it a specific function


See http://drupalperformanceblog.com/drupal-xhprof-profiling about the article.

Once you have the problem it's possible to fix it.
You wrote that you had enabled views caching: Which kind of caching output or query cache?