Let's all collect some tips & tricks for MyBB here...
- Try to use valid HTML code. MyBB uses valid XHTML 1.0 and you should always try to stick with that.
- Do not overload your MyBB with Plugins, it all takes server load and it might even get confusing for your users. If you don't want to use a plugin any further uninstall it and take care that the templates have been fully removed.
Delete the plugin file from server to have a better overview about your plugins.
- Start with the info and the hooks and after that do the plugin function. Do the activation process lateron - do the first changes manually. That makes it easier to find bugs.