I've tried adding many backlinks to a new site and adding many links (even quality ones) do not seem to help the sandbox effect.
No, new backlinks during sandbox don't help you, can even make the thing worse... but after that, they can be counted.
i have web site and i have build backlinks so fast for this web site. it get pr 6 for the last update but it's sandboxed now
To my best understanding, there is no "sandbox", but a filter that Google applies in the very competitive keywords. This filter is based among others by the age of the domain. I believe that in order to start to get traffic from Google you should try to target less competitive keywords. That will help to sustain the first 6-8 months while the "sandbox effect" still applies.