You should install Auto Cookies which will resolve many problems uses will encounter trying to log in and stay logged in. When I switched over to phpBB it was a nightmare of log in issues until I added this mod:
http://www.phpbb.com/phpBB/viewtopic.ph ... ht=cookies
After that you'll probably want to stop the spam bots from posting and registering on your forum. There is an anti-spam thread that discusses this, the following is what I've done and it's been very effective for me.
First is to get the bots from being able to register in the first place. The captcha that phpBB comes with is pretty ineffective. I had it turned on and spam slowed very, very little. That's because the bots can read the captcha and enter the correct solution, which is as effective a prevention measure as not having the captcha at all. Instead install this mod to present a challenge to people registering that's unique to your site (use you own questions, not the example ones):
Then to stop anyone who might manually register in order to have a spam link in their profile, add this mod that requires a minimum number of posts before you can have a signature or web link in your profile (number doesn't have to be very high as spammers won't be posting anyway):
Then finally hide the registrations without posts from view in the member list and from the total member count:
http://www.phpbb.com/phpBB/viewtopic.ph ... light=zero
You could do either/or with the last two and still accomplish the goal, but doing both makes everything very clean. If I look at my database and view registrations without any posts, there are none there that are obvious spam registrations. Like the posting, it seems that it's pretty much all bots doing the work and not real people. However if a real person is sent out to make a registration, it will still do them no good and as there will be few, if any, in the database it will make no difference if they're in there or not. One could periodically delete users without posts, whether or not they were intending to be spammers, as after a while people that never post probably won't ever be posting.
After these anything else will depend on what you may or may not need.