The "slave mode" style of speech is a pretty good ongoing training tool. It can look forced and artificial, but the fact that you have to plan what to say is part of the point.

The first rule is that you always speak of yourself in the third person. You may just use some phrase like "this slave", but your Mistress might encourage and reward you for being creative (and making it more interesting for her to hear) with variations like "Mistress' boy" and "this man-toy".

The second rule, which not everyone feels is necessary, is that you always refer to your Mistress as such, and never say "you".

Finally, you might be encouraged to elaborate it with compliments to your Mistress and deprecating references to yourself. So you might find yourself saying things like "Will the beautiful Mistress want more service with her bath, or may this lowly servant go finish his humble chores and come back to have the undeserved privilege of drying Mistress' awesome body?" Composing that sort of sentence regularly as an exercise in submission is bound to make an impression on your own feelings, and your Mistress will enjoy the ego stroking as well.