I was having trouble remembering tones when practicing with Duolingo because there are minimal checks that you actually know the right tones for each word or character. ToneBoard requires you to type the tone as well (e.g. “hao3” to input “好”), which I think really helps with remembering the tones. (I am also learning Vietnamese and really appreciate needing to type tones with Vietnamese keyboards.) You can use it with any app in iOS, so it also lets you practice recalling tones while texting, writing a Facebook post, or whatever.