Contests are one way but I would try and find some good posters and invite them to come to your site. Heck, if you have money to spend, you might think about giving them some kick backs for bringing people in and adding content. Content is key and so is some type of activity. People need a reason to get there.
I don't know if you have to have all those forums but you might want to cut down on some of them at least for the start. Having too many "empty" forums can be a real deterrant. When people show up and there isn't much activity they might just click-out and go someplace else before even digging into what you already have there.
You don't have to delete them but turn it off in your adminCP until people ask for them or there is already traffic available.
I have over 160 forums at one of my communities but I started out with around 10, I think. People ask for new stuff all the time but I don't add them unless there is enough ongoing demand. Forums that I do open and don't get much traffic, I will often merge or get rid of.
I would spend a lot of time posting links and information where applicable. Dig up any web directories that cover your products. Ask people to add your to their links sections, etc. The hardest part is when you start. You really have to bust your tail. Once you get some good and active members, the word will start spreading.
Rule of thumb.. Most all communities don't start taking off for a year. It took my first community a year and 4 months before it started to really blow up.
I think from December 2001 to March 2003 we ended up with about 2,500 members joining? From March of 2003 to this day, we now have 22,161 members. We've haven't been getting as many as we were lately only because of the upgrading and changes that we have had going on since January but we should be back up to about 150 new members a day and 2,500+ posts.
I truely believe that if you want your community to succeed, you can make it happen. Having a viable concept is important but you have to pour a ton of blood & sweat into it and then chances are you will realize your vision.
my 2 cents.
David