What are some ways that I can be self-deprecating about my Chinese speaking ability?
You can try the following sentences which demonstrate humility:


不好意思，我的中文水平尚浅，有待跟您多多学习。
Sorry, my Chinese language skill is still shallow, there is still a lot to learn from you.
我的中文只不过马马虎虎而已，请您多多包涵。
My Chinese is just so-so only, please bear with me.
我的中文说得不好，请别见怪。
I am not good at speaking Mandarin, hope you don't mind.


If someone is praising your language ability, you can express in the following manners immediately after the compliments:


哪里，哪里... (Nah, you think too highly of me...)
您过奖了。 (You flatter me.)