My current site has about 2000 members.
I have been an admin on sites with 5000 members and 20,000 members (It wnet under then the owner closed it without reason...)
Spam
Well, one big thing to take a look at is the databases of known spam bots/ sites, and input ALL of them into your forum's block/ auto ban feature. It will take out a lot of new members early on, but will also save you a TON of grief down the road.
As has already been stated, add some sort of GAPTCHA to your site, that just helps to weed out those spam bots that aren't on the list.
Someone you trust
Another thing to do is start off knowing at least one person who is willing to help out on getting your site up and running whom you can up to moderator/ admin. This will take a lot of the daily dealings with idiots off of your hands.
Start small
Dream in Code started out (from what Sky has told me) as a mainly web design based forum, then as members asked for expansion he obliged.
One thing I did terribly wrong with my forum was starting off WAY to broad. I tried to include Xbox, Playstation, Wii, old consoles, general chat, cheats/ mods/ tricks/ tips, and the lsit goes on. People got confused by this, and many chose not to join because of that.
Don't Re-hash
Don't create something that already exists, you need to have a community which fulfills a need people have. If you are going to create a clone of something that already exists you are going to suffer. Why should a person join your forum over one that has had time to establish itself? What can/ do you offer that other people don't?
Look Good
Don't go through all the trouble of making a site if you aren't willingto work on a skin long and hard of shell out some dough (money) for one to be done professionally. I didn't realize how important skin, layout and general asthetics were to a web based community until it was too late. I started off with a fairly simple, dark skin, but when I noticed that the new members flow had come to a total standstill I decided it was time to come up with something new. The second skin I made for the site was a greyscale one. I thought it looked nice, but it worked even less than the previous one had. So I created a third skin (which my site is currently using). While it is still grey for the most part, I have added in shades of blue and made sure the overall skin is fairly bright. It is easily visible now and uses rounded fcorners in a few areas.
I suggest taking some time and looking colors and how people respond to them. For instance (brief overview) greens and blues are used to calm a person and get them comfortable while on the site. Yellows, oranges, and reds are commonly used to excite; if you want an impulse buy or to capture the attention of a guest use a bright color that stands out from the rest of the site (like #f00 on bold text). Use complementary colors, they look better together and helpt o highlight various content portions without blinding a guest (though it is still possible to hurt your guest's eyes based off of your color choice).
Have fun and do what you love
A general rule of thumb is to have fun with your site. Don't just go off and act serious/ uptight people want to have an escape from their everyday lives, not somewhere they can come to and perpetuate their worrying.
Registration time
When it comes time for a user to register on your site don't make them input a lot of information to get an account up and running. Figure you hae 5 minutes max to get a user through the registration process before they just say "screw it" and walk away. It is common, and fairly intelegent to simply go about asking for a username, email address, password, and password confirmation. That way the user can login and start being active asap. If you are worried about a spam bot use a simple CAPTCHA in the registration form, but don't make it overly complicated.
Capture your audience
With the way the world is today you don't have much time to do anything before a person's eyes are being averted to something elsewhere. That means you need to have your site's homepage count. Figure you have 10 minutes max (TV shows normally run for 9 minutes followed by 9 minutes of commercials to continually refocus the viewer) to make an impression. You need to have the content quickly visible, and easy to find. Add a search feature to your site so if the user comes and wants to find something they type it in and are on their way. You can't afford to waste their time looking through the numerous articles you have on your site until after they've been hooked.
Update frequently
If you want your site to continually be at the front of a person's view list you need to make sure there is high-quality content posted on a regular basis. If that means you are posting a new article daily, or a few times per week that is how it will be. You can't expet for a user to view your site on a daily basis expecting new content just to be dissappointed to come back for very long. If you don't have a set time when the user can expect content, or an already established database of content for the user to skim through they will lose interest in your site and go elsewhere.
Feed Me
Adding XML, JSON and ATOM feeds to your site will maximize the people you can reach on their terms. It isn't always about making things hard for your users, why not try making it easy for them? Offering a feed will allow them to skim through your updated content and other content they are itnerested in (from other sites) without having to come explicitly to yuour site. If something catches their eye they will look into it further. It will save them time, your users will appreciate that.
Titles pop
A picture may say 1000 words, but that doesn't mean works can't speak loud enough for themselves. Many modern search engines are beginning to disregard things like meta keywords and instead focus on your content. Make sure that your news, and post titles have a keyword-rich title whilst being human readable. You will lose viewers if your titles are a jumbled mess of words that make no intelligible sense when strung together. Aim for something specific about the article, that is at the same time peeks the interest of a viewer.
