Over the last few years, my year lists and my life list have not been that far removed - my life list (ABA area only) is 577, but I've seen over 450 species each of the last three years.
This means, as you might guess, that I've covered the same general areas each year, with the odd rarity or seasonal bird thrown in.
I cover a good bit of Texas each year, and have really had to learn a lot about vocalization, microhabitat, etc. in order to expand my list. This process continues; for example, I only added Gambel's Quail to my Texas list this summer, when I finally took the time to bird in the El Paso area. Taking the time to learn about the distribution of this species was necessary in finding it. I'm still missing Sage Thrasher for Texas because I've not birded in west Texas much at all in the winter.
I suppose that this might be considered a depth-oriented approach to birding, but it takes place over several states (usually at least Texas and Idaho, and a few points in between), so there's some breadth involved as well.
I'm trying to say that there is, no doubt, the possibility of big numbers with the addition of new birding locations, but even within each location there is depth. Finding Red-necked Grebe or Say's Phoebe in Idaho is not an everyday occurrence, for example.
I look forward to finding Eastern Towhees, Fox and Harris's Sparrows here in the Houston area each winter, since they're not just sitting on every other fence post. There are easier places to find each species, but finding them here is more satisfying for me. Again, learning something about the habits of the bird helps immensely.
Even when going to never-before-birded areas, learning about the vocalization, habitat, and seasonality of species will help you add even more of those new species to the lists you keep. It's true that some species will just fall into your lap in new locations, but there's more than meets the eye in most situations.
Steve in Houston