Seaglass-
I'm not sure what, in general, makes someone competitive. I know what our own institution looks for, which tends to have a high number of applicants. Aside from all the regular stuff, what it seems to really boil down to is what Dr. Sierzenski said, personality!
Our new and most recent fellows have all rotated here, thus the directors were familiar with them.
I have found that the best thing, aside from simply scanning alot, that has helped me out is the Ma "emergency ultrasound" text. It is surprisingly easy to read, has great pictures (crucial) as well as simple how to instructions. (ie, I had a patient with very difficult access, refusing any EJ or central line and I had read about u/s guided basilic vein. As he was in CHF and SLNTG were not working, I cracked open the Ma, read it, looked at the pics and viola! Access. I have done it several times since and it works great.
So its a very helpful text that is easy to use. While direction is definately better, you can still really do a lot with the Ma book alone. There is tons of research to be done as well. I did a small research project as an intern and am working on a larger one now...
I am planning to sit for my RDMS in a month or so. Wether or not this makes me competitive is another story. What I do feel comfortable with is having the skills to go and encorporate U/S in my day to day practice.
One has to think seriously as to why you want to do an ultrasound fellowship. It is much more than just learning to scan. Its about CQI, department establishment, learing the details and subtlities of running an ultrasound department.
Chris Fox, MD in california also has a great program, similar to Dr. Sierzenski's, where scans are reviewed on video. Definately something I would recommend if you are going to spend time doing an elective.