It is Memorial Day weekend. Be sure to take advantage of Ancestry's offer of free military lookups! You can find it here: http://www.ancestry.com/cs/us/memorialday2014.
You will need to sign in to Ancestry to view the records but you can do this with a free account. Enter your search parameters. As always, start with a more wide search. Put more details in if you get too many results.
In my case, searching for Leo Miller, I had to enter his birth year to narrow down the results. In addition, on the left-hand side of the screen, you'll see a list of the military collections that are part of this free offer.
After clicking on the U.S. World War I Draft Registration Cards 1917-1918 link, it narrowed down my search quite a bit.
Here is my grandfather, Leo H. Miller's World War I draft registration card.