There are a few ways to go about it. First, make sure you've taken the video in 720p. I could be wrong here, but I believe you have to take the video with the phone sideways (in landscape mode) in order to get the true 720 resolution.
Correct me if I"m wrong.
But you can upload a few ways.
First, you can copy your phone to your computer and copy it to your computer by the old "drag n drop" method. That will preserve your video and not compress it.
An app you can use is PixelPipe, that will allow you to upload HD videos up to 200MB to many many different sites such as youtube, picasaweb, facebook, dropbox, etc.
Its free and it works, but has that limitation.
Hope this points you in the right direction.