In addition to what was stated above, if you don't have a United MP Visa...get one.

With the MP Visa you can use "Choices" (Note that the "Choices" balance you have is from the total eligible purchases you've made with your MP Visa...search for it on this forum there's a whole thread on it)

Pretty much with using MP Visa "Choices" you can buy a ticket for $400 on united.com (it has to be purchased on united.com to be eligible...valid on any carrier) You can then elect to use 40,000 miles, ($100=10,000 choices miles) to get credited the $400 back to your visa and you still earn EQM's on the flight. (It's like using your miles for the ticket and still earning EQM's for it)

You can also use it for partial payment....(i.e. use 20,000 Choices for a $200 credit towards your $400 ticket purchase)

Hope this helps!