With careful planning, your layout can be built in a way that you can model several different eras by just changing a few details. For example, the scene pictured here shows a layout set in the 1930s, but it could easily be changed to show a layout set in the 1950s by simply changing the vehicles on the road. With newer vehicle models, it could also represent an even later era, perhaps the 1980s by also changing the sign on the structure and adding a stop sign at the grade crossing. If you build your layout with removable structures, you can replace Victorian architecture with Art Deco architecture to further enhance the era change.
It's said that "the only constant is change," and this tenet is evident in a myriad of ways in the rail transport industry. As model railroaders, we have an equally large number of ways that we can show evidence of changes on our layouts. Here's an example that I saw on the prototype over the weekend. The location in this image is Black Earth, Wisconsin, a town on the former Milwaukee Road line between Madison and Prairie du Chien. The mainline is still in use through this town by Wisconsin & Southern, but as we can see in this photo, the facility across the street used to have an industrial spur leading to it. The narrow strip of asphalt in line with the building covers the former grade crossing. Notice that the patch over the track location is slightly darker than the surrounding pavement? On a model layout, such a detail can be added to almost any street, even at a sharp angle to the layout edge, and as we see here, we can add this detail not only in the large cities, but in the small towns too.
When you get down to the smaller scales, it can become quite difficult to hold onto a figure and paint it at the same time. In N scale, especially, trying to hand-hold a figure while painting it will often end up with painted fingers instead of figures. The solution to this problem is a simple one. Paint the figures while they're still attached to the sprue; then after you cut them from the sprue, a quick drop of paint at the connection point finishes the project.
So by now we've all heard that putting a mirror at the end of a yard can help make the yard look much longer than it actually is, right? Well, be careful on what you place next to that mirror, because everything will be reversed in it. The model in this picture is on display at the National Railroad Museum in Green Bay, Wisconsin. Sure, the mirror makes the small scene look much larger, but seeing ACME reflected in the mirror as EMCA detracts from the overall effect. However, this property can also be used to your advantage; the late John Allen placed mirrors inside a building in a box shape with two vehicles within the box. The vehicles had different paint colors on each side so when the viewer looked in through the building's window, it looked like an infinitely large auto dealership.
If your model railroad junk box looks anything like mine, you've got a bunch of track scraps that will never see a rail wheel on them again. Here's something that you can do with them. Rust them up and simply place them next to similar track pieces that are in use on your layout. The prototype railroads will often stage complex track pieces next to their replacement locations, like this diamond frog I found in Muncie, Indiana, when I attended the Midwest Region Convention last year. Old rails are also often left next to the tracks for a while when they are replaced, so you can use plain rail sections this way too.
When you start your backdrop painting, remember that you don't always need photo realism for rural or mountainous scenery. Often just a rough shape of the mountain in an appropriate color palette will be sufficient. Closer mountains will have colors that appear similar to your layout's scenery. However, keep in mind that distant mountains on the prototype appear in progressively bluer shades until the mountain is just a "purple mountain majesty" in the distance, so you don't necessarily want to use the same colors as your foreground scenery.
Here's another source of N scale vehicles that you may be able to find at your local grocery. The "Bonus Game Token" scales out close enough to N scale to work, even in the foreground (after they're painted, of course). There are 30 models available of different US prototypes from the 1950s to the present, but they're a limited production, so get them when you see them.
Avoid positioning your backdrop images too high. If the horizon is above eye level, it won't look right to a person standing in front of the layout, and it especially won't look right when you put your camera down at track level.