if a thread has value it will be indexed, the more searched for keywords in the post dictate if googles is searching the phrases. so doing nothing will have the same effect as doing everything in some case's
posting a lot of threads create spider interest, the more they visit your site the more and faster the threads will be indexed.
A good way to get spiders to visit you is to tweet your posts , installing a mod like
Boofos spider in forum home is a great way of seeing how many spiders and what spiders are coming.