Study film-making as a film-maker, not an editor. Cutting is easy. Editing is hard. Because you said “better editor” not “how do I edit” let’s focus on what might actually make you better, rather than simply teaching you software skills. Probably not what you meant, but think big.
The editor is the last craftsperson in the chain. They make or break the show. I would argue the the editor is as important as the director. I might be biased, but if you can’t communicate with Directors as an equal partner, you will never be a good editor.
AFI, University, Books, local unions and film organizations. Find classes at take them.
Edit. Try to replicate what you see. Not visually, but artistically. Practice.
Walter Murch, In the Blink of an Eye (book). I suggest also looking him up on IMDB and watching his classics first so you can see how he applies the concepts. Godfather and Apocalypse Now would be the obvious films to start with.
Study other editors. Check the editor of every program you watch and then look for other works by them and try to understand the mark they’ve put on the work. It can be hard because editors, especially very good ones, are chameleons, adapting to the intentions of the director, but learning to see the mark they leave is invaluable.
Study visual composition. Selecting the shots and understanding why they work is half the job. The other half is timing. While editors rarely create the visuals, except for graphics, knowing what works visually, and why, is critical. If you can’t tell a good shot from a bad shot, and you don’t understand what the photographer or graphic artist is trying to accomplish, you will never be a good editor, much less a great one.
Okay, I neglected sound. If you want to be great, you need to understand sound, and particularly, music. I suggest studying modern Hans Zimmer (composer) movies if you really want to understand the power of sound. Dark Knight is amazing, edited by Lee Smith. As previously mentioned, many directors and editors stick together, Christopher Nolan and Lee Smith, Scorcese and Schoonmaker.
As far as online help goes, I like Moviola.com. It’s free and it’s more focused on improving your general artistic skills than your simple software skills. And if you are looking for simple, direct, practical training, Larry Jordan is the man. There’s also Lynda.com which has quite a bit of training and is available for free through some library systems and many schools if you are a student.
And finally, Masterclass. Not cheap up front, but if you want to listen to the best talk about their craft, it’s a great deal in the long run. Scorcese and Hans Zimmer both have classes.
Good luck,
tod