Well depending on what you're going for (which you said sassy) Sassy can resonate anywhere it's just mainly in the way you pronounce things. Of course tone helps, especially if you're going for a higher verses lower sounding sass.
My tips to you are to pick a voice actor/character and try to mimic that certain sound you're going for because everyone probably thinks differently in sound. I'd also focus on pronunciation, because you can have a high voiced character still sound like you unless you give them a different, personality, like feel.
Also, this is a no brainier but just dont stress your vocals. If you're having trouble switching voices entirely, I'd try fooling around every once and while trying to produce different types of voices.
In my personal opinion on "sass" it tend to be more nasal with over pronounced everything, like you're being condescending sounding or sarcastic