The only way is to keep building quality backlinks and by being patient. It happens to almost everyone at some point.
stop some days all your spamming activities and also daily make 20 or 30 good quality link which have high PR
Building quality backlinks helps to escape from sandbox.. You can start link building techniques like press release and social bookmarking.
you have to be patient. My site was sandboxed when I launched it for around 5 months. After that, I started ranking very well for my keywords so just be patient and keep doing your SEO There is nothing you can do about it
Just focus on building links to your site and update the content regularly. and you will see your site out of sandbox.