1. What I do is create lots of ad groups which each target a specific area of the forums. I have an ad group for each individual forum. Then, for each ad group, I create about 3 highly targeted ads which incorporate keywords into them. i.e. an ad group to advertise the PHP forum will use words such as php, zend, etc. in the ad copy. I get a very low CTR but of those who do click, 5 to 10% of them go on to register on the forums. So I'm happy with a good conversion rate.
2. I'm not sure right now as google is in the middle of some weird type of change to their ranking algorithm. Supposedly PR isn't being weighed nearly as much as it used to. For the most part, better ranking in the SERPS can be achieved by getting more backlinks and lots of em. Expect a decrease in traffic over the summer however.
3. Go on an agressive link-finding campaign. Visit other sites with a subject related to yours, and send a pleasant email over to the webmaster saying that you like their site and would love to partner with them. Try to make it a habit of sending out at least 10 of these e-mails per day for awhile. Now suppose your site is about "widgets". Do a google search for "widgets directory" and "widgets link swap" and "widgets links" - odds are most of the sites that come up are ones that are on your subject matter AND include a link swap directory. However, it's best to swap links on actual content pages instead of in directories.
4. Traffic always declines during the summer and weekends. You may have been blogged by a new site or such where a link to your site appeared on a high PR page for a small period of time. During that time, your rankings went up and you got more traffic. The internet isn't a constantly changing place ;) Things change, so appreciate what you've got when you have it :)
5. I use AdWords and try to find as many link partners as I can. Getting backlinks helps deliver traffic directly from similar-content pages in addition to raising your ranking in the SERPS.