Maybe you're starting from an Undec & Western model or maybe you're just weathering a prepainted model. Either way, you've got to do something to get those extra NMRA Achievement Program points.

Are you building a model that includes tinted windows? If you know anyone who deals with professional lighting equipment (think about the people you know at the local performing arts theater), you may have a source for low cost window tints. This image shows a sample pack of light gels from Roscolux. A light gel is basically a colored filter on a thin sheet of plastic. When the light gels wear out or get damaged (like if there's a hole melted in the center), they need to be replaced. For model building purposes, the rest of the damaged gel still contains enough material to use for your window tints so ask your friends to hold on to the damaged gels for you.

Unless you're modeling equipment straight out of the paint shop, it's OK if your decal is chipped. It happens on the prototype too, like this UP locomotive passing through Rochelle in 2005. A lot of railroads use large scale "decals" of their own to apply uniform lettering and numbers on their equipment, and their decals chip just like ours do on our models.
This tip could go just as easily under lighting as it could under model photography. Be aware of what kind of lighting you're using on your layout. In the same way that incandescent bulbs can cast a yellowish glow, most of the fluorescent bulbs will have a green tint to the light that they give off. When you're painting your models, paint them under the same lighting conditions as your layout. If you're photographing models, check your camera's manual on how to set a custom white balance to compensate for it.
Forty foot long boxcars are getting harder to find on the prototype. They're still out there, but just not used anywhere near as often as their longer 50' or 60' brethren. So, whenever I see one, I make sure to get a photo of it. This car, ONT 92065, is one that I saw while on vacation in Northern Ontario in 2003 (yes, with a much smaller camera than I have now). Notice the rust patterns on the car side and how they differ between the left and right sides of the door. On the right, where the door slides, there are horizontal rust streaks from the door scraping the car side (the side sometimes bulges from the load pushing on the wall), while we see a much more scattered rust pattern to the left of the door.
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.
When equipment is sold to another railroad, the buyer will often paint over the car with the new owner's official colors. Sometimes, the original owner's logo and paint shows through, giving us a clue as to the equipment's heritage. The same effects can be seen when a railroad upgrades its official paint scheme as can be seen here on this former Chicago and North Western Railway hopper. The old and larger CNW logo outline is clearly visible under the new and smaller logo, which thankfully has not been itself painted over yet. This car and another of CNW heritage were spotted behind the MG&E power plant in Madison, Wisconsin today.
Sometimes when a graffitist sprays a freight car, he will paint the reporting marks in a location that doesn't interfere with the main part of the tag. On this car, it looks like the same white paint was used to reposition the car number as was used in the tag. From the patches of blue bubble shapes above the tag, it also looks like the black and white colored tag is covering up a more colorful tag in the same location.

Rust streaks often start at points on a car where two pieces of metal intersect, such as where the roofwalk supports meet the top of the car or along the weld joints in the side panels on this covered hopper. From the origin point, the rust normally flows downward, pulled by water and gravity.
An initial wash with diluted water based acrylics makes your cars look like they've been on the road for a few months. The color collects in the crevices and makes the details stand out more clearly. The photo shows a before and after view of this technique applied to a pair of N scale boxcars.
One thing that I always find fascinating about graffiti on railroad property is how the railroads deal with it. I've heard stories of graffitists getting arrested or chased off and then the equipment moved to the paint shop for a patch job, but I've also heard stories of other graffitists who were simply advised not to cover the numbers. Inevitably, many freight cars these days will have their reporting marks covered by a graffitist's piece. Here's one of the more interesting solutions that I've seen recently; the number was repainted with spray paint in a way that makes me think it could have been done by the graffitist himself. On our model railroads, this gives us another way to renumber equipment without getting out the paint strippers. However, for cars used in operating sessions, remember to get the reporting marks on all four sides so your operators can spot the cars correctly on your layout.
Want to add a little variety into your locomotive fleet but still keep with a prototype modeling scheme? Try adding a leased locomotive to your roster. Leased locomotives will often be painted in the colors of another railroad, sometimes still wearing the livery of a fallen flag. In this photo, we see DPGX 2000 at the Wisconsin and Southern Railroad locomotive facility in Janesville a few years ago; this locomotive is painted in an early scheme of Dakota, Minnesota and Eastern Railroad.
Use a light-colored paint to recolor some of the roof hatches on your covered hoppers. This is an easy way to simulate a replacement roof hatch without actually replacing them. Would you believe that the photo shows N scale models?