That refers to a specific item. ie: that size six shoe. A visual image is to think of the two ts as fingers pointing to that shoe. If that can be removed from the sentence and it does not affect the meaning, you can change it to which.

Which refers to generalities. ie: which set of shoes is general and is not talking about a particular trait of shoe.

Do you have a better recommendation of how to keep them separate? Please share in the comment section of the blog.