Here's how I trained for the Elite Four:
First, go to the spa on the Sevii Islands. Come out. There should be two running guys, a team of fighters, and a girl in the top. Beat them, then afterwards go into the spa and heal up. When you come out, your VS. Seeker should be ready to use. Just keep beating all of them over and over.
The easy way, though, is just to capture the three legendary birds. They should all be able to make most of the E4 easy, except probably for Bruno. Articuno could probably take down most of Lance's party itself.