Buy a domain , without a domain it leads users to think that the site will die sooner or later as alot of forums hosted freely or using a free domain tend to last not long..
If you have a proper TLD like .com, .net, .org then it shows people that you are willing to spend money on your forum and you probably won't just give up on it as easy as you would have if you had a free domain.
Also not being on a free host shows that you are dedicated to your forum more as again you are wiling to spend money and a lot of time getting hosting and installing and updating and modifying the forum yourself so again it does show to your members that you won't give up on it 2 months down the line.
Really it is about showing you members that you care for your forum and that it will stay active. Moving to paid hosting and getting a domain was the best thing we have done here at FP.
We advertise in many ways... Mainly on other forums and other signatures. We also get alot of members from Google and other search engines aswell as word of mouth.
Just because you don't have a domain certainly does not mean you are not putting effort in. A domain is very useful to have but not having one doesn't mean you are not putting effort in, as long as your forum is doing good and your members are happy then that is enough to show your are putting effort in. ~smile~
If you don't have the money to buy a domain or for whatever reason some forums might hold a contest and the prize is a free domain for one year. I have never tried DNTemple but I know quite a few who have got a free domain from there.
There are big sites on free hosts as well. You just got to get exposure all ways possible, youtube videos, create tutorials, etc. Keep pumping out articles! Create/Find content, then share the content which will then lead to more people to your site.