Members should be considered the same as you would consider a guest in your house. If the house is dirty, doesn't look good and has angry people in it, then the guest will want to leave. Well, it's exactly the same with forums. You should always keep your forum clean of spam and trolls. These spammers should be banned and their posts deleted, same with the trolls. Also, you need to make your forum look good. If you walked into a house which had stains on the carpets, rips in the settee and marks on the curtains, would you feel as welcomed as if you walked into a house which is polished, cushions plumped and hovered carpet? No.
Looking good is the thing. Make sure you have a nice, appealing theme which isn't too light or too dark. Make sure your logo is appealing and memorable. Make sure your images are set out well and you don't have too much blank space.
Also, remember what I said earlier about being polite to your guest? Welcome them with a nice Personal Message. Don't use an automatic message sender, but each time a user signs up send them a personal, polite message welcoming them. If they make a welcome topic, reply to it promptly with a personal reply and make sure that your staff members do the same. All these things will make a user want to stick around more, and thus means more activity on your forums.
Oh, and one last thing. If a user breaks a minor rule, don't go on a power run and warn them straight away for it. Take the time to send them a kind PM explaining what they did wrong, and mention that you'd appreciate it if they reviewed the rules. Tell your staff to do the same in such situations. Only when you're dealing with repeat offenders do you need to be dealing out official forum warnings.
Good luck on your quest for forum guruhood!