It is very difficult to implement without some bucks;
even some big sites/forum which has put much investment of money to promotion before getting tons of active members.It is only a game of figure ($$$)
I suggest you find more part time job and get some bucks to spend on the promotion.

the way in which you tackle it is specific to your niche, but some general points:

1.  this would entail the same techniques you'd use to get people to any type of site.  blog commenting in the same niche, sig linking, leveraging high traffic sites like yahoo answers, targeting specific keywords and working on your google ranking for those keywords.  do everything you can to gain and maintain a good se positioning

2. keep some juicy nuggets hidden from guest browsers and only available to registered users.  this can get people to register.

3.  keep posting open-ended questions and/or posts.  make multiple users and make some of them seem controversial - getting people's backs up can often enncourage them to post although you do not want a flame war on your hands so it needs to be subtle.

4.  make sure every comment gets a friendly reply.  if you have worked so hard to get people to your forum, you should not leave their posts (no matter how lackluster or trivial they may seem) unanswered.

1.  In order to get people to yoru forum you need to get the word out.  If you can spend even a little money to do this it can help.  Otherwise get the word our about your forum.  Tell everyone who you think might be interested about your forum.  Sometimes you are able to announce your new forum in another forum (as long as it isn't direct competition). 
2.  Ask some friends and business associates if they would mind helping you start up a few conversations in your forum.  This is a way to become developing the forum before attracting visitors to the forum.
3.  As an adminstrator you need to be active in your forum as well.  It shows members that you care about your forum and will work hard to make it grow.
4.  conisder having some contests.  You do not necessarily nead money for this.  You can offer things such as free advertising.  And again keep the content flowing so that people have a reason to keep coming back.

PHPBB or SMF or any other forum setup will not help you and there is no difference. I think it depends on your devotion to your forum, it depends on how many hours you can work. It is a continuous process and can't be stopped.