Several days after my site launched, Google indexed ~150 pages of it. Then it indexed up to 1000+ pages. But soon the number went down. Now it keeps a ~150 pages. What should I do?
Is the content duplicate ? if yes then there are low chances of it getting indexed. You can try creating a sitemap and submit the same via Google Webmasters tool. Next you can try getting some backlinks to your site so that google comes to your site again n again. Hope this helps.
Backlinks always help. A Sitemap will get your pages into their index sooner than without. Time is your biggest enemy... The sudden spurt, then dropping off is not uncommon for Google. Google has several spiders that crawl your site. One is like a scout that finds URLs and moves on quickly, another is like a vacuum cleaner and takes it's time to scour every inch.
Get yourself related backlinks, and some patience. I don't k now how long is it since your site was up, and if it were sandboxed or not? It is possible that your site is sandboxed, also consult what your google sitemap says about your site?