Since this thread randomly opened back up after eleven years:
On the Ground
FM: The higher up you go, the better the distances you can receive, thus you should check out your local mountain pass to see what conditions are like. Folks go up to Pikes Peak, Colorado to easily cross the 150-200 mile marker, and some parts of Oregon can push 300 miles on a regular basis. Just maker sure you are not being blocked by other mountains.
On the other hand, flat-land is golden for TROPO, and Arkansas is known for regular ducts to Texas, Missouri, Nebraska, and Tennessee. Here in Wyoming, tropo is rarer and shorter, but can impress, such as the 400+ mi duct to Amarillo on June 9th of this year. Hills will block tropo, hence I get all my tropo to the east and the south because of the mountain range to my west.
AM: The key you are looking for is "ground conductivity". I agree with other posters on this site that the Dakotas, Nebraska, and Kansas are awesome for this. Example: KFYR (165 miles) and WNAX (307 miles) are reliable and strong in Rapid City, SD. I could even, with a sensitive portable, pick up 740 KNFL during the daytime (approaching the 400 mile mark) while in RCSD.
Cold locations can help, too. There is also the salt oceans which are monsters for conductivity (5000 in the Atlantic vs 30 in the Dakotas), but that isn't really land.
In the Sky
FM: Be on the look-out for E-skip, which runs in the Summer for most of the US, but can be heard any-time of year if the conditions are right. Some research shows it is correlated with the sunspot cycle/the intensity of the sun. This might help explain why there are more openings in the SE United States than the Pacific Northwest. But, the true winner has to be the Pennsylvania to Tennessee region, and the Nebraska to Texas region, respectively for most-common pathway. Everyone's experience with skip is different, though, and one location does not necessarily have a higher chance than one a hundred miles away. Although, again, being in a warmer location can help, location is not what will increase your skip, it is equipment. Get an SDR and have it recording. Sign up for e-mail alerts, monitor the online skip-log, and whenever you feel there's a chance, check your radios!
I should add, there is Meteor Scatter, which can happen anywhere, but not necessarily anytime, but most Dxers grab a dozen or so every morning. (I however, haven't been so lucky). The best time would be around the Perseids. Plus, you have Auoural Skip, Equatorial Skip (a slightly different animal than regular skip), and other flavors to try.
AM: On land specifically, conditions are helped by a low-noise environment, and where the skywave signal can be enhanced by ground conductivity, hence the plains win, again. But, for best actual skywave, polar regions are good, and apparently areas with lots of oceans are even better. As
@gar hi can tell you, being in Hawaii with water on all sides allows for some unique catches of American clear-channels, and probably even some Australians, etc. He has nailed WABC before at 4,700+ miles (How in the world, gar?) And as
@SomeRadioGuy will tell you, his post in McGrath is huge for trans-continental DX with big distances and strength, such as Radio Iran at over 7,000+ miles away. In addition, there are Polar SDR's set up in Iceland for a reason, and the story goes that 500 watt KGAB (Cheyenne, WY) was picked up in the polar region that is Finland (I don't even want to imagine the distance here).
Added bonus: For shortwave, if you are into that sort of thing, the Eastern US near Indiana seems to be prime for US based and Cuban stations, but even here in Wyoming, you can snag Radio New Zealand (7,000+ miles) with ease, and of course, the mighty KBC (Germany, 5,000 miles) at times. And again, the coasts win big.
Best of luck to you!