i always think quality of information and ease of getting to (and enjoying getting to) that information is the best advertising you can get.
for example, it doesnt matter how much you advertise, if you site is not well thought out or is hard to access what the user wants to access or has a drap setup--no one will repeat visit (and repeat visits is whats most important when maintaining a forum).
if your site is already well conceived, well thought out, and the setup is intuitive, informative, and has an attractive appearce (layout) then all you need is little advertising and more time.
since you mentioned that your website is in holland--is it mainly for local use? if so--then 150 users is NOT bad. celicas are few and far between, and celica users that frequent celica websites are much more few & far between.
for instance, ecelica has about 6,000 members--on a more or less global basis (mostly u.s.--but we do have quite a few international users). obviously this is an extremeley small percentage of celica owners, but is a good percentage of celica users that frequent celica forums (if you can call most of our 6,000 members 'frequent').