How can I force a line break in rmarkdown's title?

Try using a pipe | in each line:

---
title: |
  | Veryyyyyyy  
  | yyyyyyyyyyyyyy Looooo  
  | oooooooooooooooo
  | oooooooooooong 

author: "Foo Bar"
date: "6 March 2015"
output: html_document
---

For a centered, multi-line title:

---
title: |
  <center> Veryyyyyyy </center>
  <center> yyyyyyyyyyyyyy Looooo </center>
  <center> oooooooooooooooo </center>
  <center> oooooooooooong </center>

author: "Foo Bar"
date: "6 March 2015"
output: html_document
---

I know that this is an old post, but I wanted to chime in with:

# A Title of some kind
<br><br><br><br><br>  

# Another title that has 5 blank lines above it

Note that you need 2 spaces on line 3 or the title will not display properly in Rmarkdown::html_document. This solution is a little hard to find, hope it helps!