将文本对齐到div的中心

I have a div thats about 200px x 40px. Sometimes this block will contain a single line of text and other times it will contain two lines. In the case that it contains 2 lines of text, I have adjusted the line height so that it looks balanced inside of the div. However, in the case that there is a single line of text, the text lines up with the top of the div, leaving the bottom empty. I have a div thats about 200px x 40px. Sometime