Just make sure everytime google crawls your site there is no content, than it will start crawling faster to check if theres new content
Really there are all the things already listed, but comes down to frequently updated content..hence why many forums get crawled so quickly.
I update my site every day my alexa is also better than before,it keeps updating every now and then but PR is just the same. I have used all the above methods.
sitemaps are useless. we've done some experimenting and it absolutely doesn't matter whether you submit a sitemap or not. more on the contrary - in one case, submitting a XML sitemap to Google resulted in the crawler NOT crawling most of the site properly as opposed to another site working off the same MySQL database where we didnt submit a sitemap and it did crawl and index everything. otherwise: - lots of backlinks to deep pages - heavy internal linkage - tagging and search result pages - enough unique content on each page - fast loading speed are some more factors.
You could submit them to social bookmarking sites like Reddit and Digg, since Google crawls these sites regularly, having your site on them will equally get it crawled and indexed faster