1. (Easy) Use built-in update system. Try not to delay forum updates. New update? Apply it. You can do it with one click from admin panel (well, actually two clicks). If you miss few updates in a row you may need manual update (upload files, runs update scripts and so.) I don't think that you love to do manual updates if you can do automated
2. (Medium) SMF has a very flexible permission system. And I have to say that it's quite easy to screw it up. Be very careful and check actual result. I recommend to have few additional forum account just for permission testing.
3. (Hard) Try to avoid any direct forum source code modification. The best way to add some custom feature is to develop a
custom SMF mod. It may be quite hard to start with SMF mods but it will repay later. Try to loot at any existing mod for start. Why it's bad to just change something in forum's code? Any update
may turn into a problem. And it's very easy to loose track of custom changes. With mods you can easily revert or redo nearly all changes with few clicks. And sometimes you may decide to re-install fresh forum on a new place. And with mods it will be MUCH easier to restore your custom features on the new place.
4. (Medium) SMF has built-in caching system. If you can install Memcached on the server it's usually nice thing to do. DB query number will be noticeably reduced for popular pages/sections.
5. (Easy) SMF 2 executes some maintenance tasks on schedule. You can take a look
here. It better to configure some rare but heavy tasks to be executed ate the time when there is lowest load on the site.
Those tips are quite random but that's all I can remember for now.
P.S: Sorry for bad English. It's not may native language.