Also, it's VITAL to always keep your board software up to date. The problem is that each time you apply an update you have to re-apply all your add-ons. Hence, add in as few as you can get away with.
I disagree. There is an add-on called EasyMOD that can apply the updates and extras for you, without the need to reapply the extras themselves after a boards update. It has worked for me no problem.
Ok, then I'll make two more points for why you should minimise your mods:
1) EasyMod is not perfect. Not near. It can and does mess up. The more mods you add, the more likely it is that easyMod will not find it's hooks (because you've modded them out of existence). The older your mods, again, the more likely it is that easyMod will fail because it can't find a hook because an update changed something.
2) Many mods are really badly written. At best many add in serious performance problems. I've seen some mods that are so inefficient with their database calls that they increase the load on the server 10 fold and slow it down too. In fact I've seen mods kill servers by being so inefficient. As if that's not bad enough bad PHP code is also a security nightmare. The core of PHPBB is pretty solid by now and when problems are found they are patched quickly. The same is not true of mods.
Bottom line, mods are dangerous and can get very messy. Choose carefully and choose the minimum you need. It's very tempting to just go mad and apply mod after mod after mod. My advice is don't. Sooner or later you'll regret it.
Bart.
  