According to the panel (Sarah Fix of Blend Images, Ellen Boughn and Jonathan Ross), you want repeat sales, so your business strategy needs to be based on how you can make that happen. Agencies and clients want to see that your sell-thru rate is high, and keeping your selects down will give the images you do submit a better chance of a higher sell-thru rate. So when they search your images or want to know more about you as a stock photographer they can see that you make money.
Sarah Fix made some great points about knowing your competition. Echoed by Jonathan Ross, they both spoke about looking at agencies and other shooters and identifying holes in the coverage. You have to be smart and strategic in approaching your career. But isn't that true for anything?
Jonathan also made another point worth considering: think about the way you shoot and how well your image works as a thumbnail. If you open the front page of any agency there will be a grid of thumbnails. The ones that speak most clearly at that size are the ones people will open. So shoot cleanly and with clarity--crafted quality. Make your work stand out.
If you shoot travel and are wondering how to do more, and wondering what to do when there's no money to travel to exotic locations, remember this: You can shoot in your own backyard. Someone is interested in where you live, so take what you know and put it to use to build a great, intimate library of images that no one else would have of the place where you are. You know it best.