The most useful change I made was the button under the reply box, changing "upload a file" to "Upload a file or photo" because people were forever clicking on the picture icon in the toolbar and getting confused when wanting to upload photos (they didn't all associate files as also being photos).
Then I changed "Reply" to the right of each post to "Quote" to make it clear that it quotes too (to help cut down excessive quoting of almost every preceding post).
I also did some video tutorials on adding video to posts, adding images, checking new content. You can't beat being shown by video in the actual forum you're using.
Finally, used the excellent add-on which welcomes new members with a PM - so not only giving them a personal welcome but also running through what to do next and where to get help.