Generating detail maps for your mesh
Once you have a finished a detailed sculpted mesh and you have imported your new UVs, there are a couple things to consider before rendering. If you are doing models for game assets or plan on making the object small within your scene (like my icicle), using a multi-million polygon mesh is not a good option. What you want to do is use a low-res subdivision and use a texture map to describe the detail. This is faster to work with in your scene (less geometry) and offers more control at render time. So then you need to decide what's the best way to keep your detail: a normal map or a displacement map. Detailed bumps in game engines are achieved through normal maps, an RGB image that tells the video card what way a pixel should look like it's facing (the normal is the direction):
Normal maps are better than traditional grayscale bump maps since they contain 3 times the information (RGB) where a grayscale bump is merely up or down. Unlike displacement maps (which game engines can't use), normal maps don't actually displace the geometry. A normal map is a rendering trick, so close-up renders will show this lack of physical displacement:
The trade-offs in using a displacement map are that it uses a lot more memory than a normal map and it's much slower to render since it's actually creating more geometry at render time. If you are using a 32-bit renderer and have a ton of objects with large displacement maps, you're going to hit the memory ceiling pretty quickly and your renders will crash.
Once you've decided to use a normal map, another decision needs to be made: what subdivision level from your sculpted mesh should you use as the base. This is why it's important to use a sculpting program that can preserve all subdivision levels and generate normal/displacement maps relative to any level of subdivision. If the really boxy cage mesh was used for your normal map, the edges will look angular even from far away. You see this a lot in games—the model looks super-detailed thanks to the normal map, but the edge shows a transition of a few distinct polygons. With displacement maps, it's less of a concern. But you will still want to use a higher subdivision than your base cage for a displacement. Displacement map workflows vary between sculpting application, modeler, and renderer so I'll have to leave that part up to you and Google.
Using 8-, 16- or 32-bit normal or displacement maps
The rule here is simple: the higher bit depth you use, the smoother your bump transitions will be. If you're making a mountain with a displacement map, 16-bit should be the minimum depth used, or you'll make a hill with flat steps along the height. 16-bit is usually plenty of information for a displacement, but 32-bit maps offer a distinct advantage for renderers that support them: relative height. If you scale an object with an 8- or 16-bit displacement map, the displacement doesn't scale with it (unless you're smart and create an expression for your texture node) but 32-bit maps scale with the objects so they are easier to maintain within a scene.
Advanced technique: ambient occlusion maps
If you're feeling a bit overwhelmed at this point, feel free to come back to this section later, since it's more of a finishing technique than something you must do or else everything will fall apart. But ambient occulsion (AO) is an important thing to learn, because it introduces two key concepts in 3D and compositing: passes and baking. Both will be talked about more in the rendering and animation section later.
Ambient occlusion is the phenomenon of how light attenuates (drops off) in crevices or between objects, and it's a key component of realistic rendering, whether for film or stills. While some renderers like Maxwell Render integrate ambient occlusion into the ray trace/global illumination calculation, most need to do it as a post pass. When the separate AO pass image is done, it is composited with the base render by multiplying it over the global illumination pass:
The base render:
The AO pass:
The composited result:
AO maps are sometimes called “dirt maps” because the results look like dirt in cracks. Popular renderer VRay prefers this dirty naming scheme:
Since ambient occlusion can be slow to calculate, it is often “baked” into model textures so it only has to be calculated once. For game engines that aren't advanced enough to have realtime ambient occlusion, baking AO is still the norm. If you're making a game for an iPhone, you will definitely want to look at baking occlusion since it's a free way to fake good lighting and detail so there's no framerate hit.
AO baking can be done either by your modeler/renderer, a sculptor or retopology program. Some non-modeling/rendering programs that do AO baking are ZBrush's ZMapper plug-in (it's called a cavity map), XNormal, Topogun, 3D-Coat and Mudbox. Just keep in mind that if you do it in a modeler/renderer, some require you to create a light dome for even lighting of the ambient occlusion calculation. This is why most people prefer to do it when they output displacement or normal maps with their sculpting or retopology app, since they don't require an elaborate light rig to make.
Since AO maps often need tweaking to falloff and darkness, using 32-bit maps can save you the work of regenerating the map. Adjust the curve in Photoshop as harshly as you like—it will still be better than an 8-bit texture. When you get the baked AO map, just open it in Photoshop and set your AO layer to multiply over your diffuse (base color) layer. Import the new layered file as your diffuse color texture and you're good.
Hint: use TIFFs
You should keep your baked ambient occlusion pass on a layer so you can adjust the contrast and layer transparency of the occlusion pass later on (it can often look too dark). Even if your 3D program doesn't support Photoshop files, the more common TIFF format can contain all the layer information of a Photoshop document. Even Photoshop-specific features like adjustment layers or smart objects will be retained in this amazingly future-proof format.
Program considerations for sculpting workflows
Many 3D programs contain sculpting tools, but few meet the creative and technical demands of a truly professional sculpting program. Maya's Artisan tools are basically useless for anything but giving a bit of organic featuring or smoothing to models. blender lacks alpha masking so you can't get textured strokes. modo's sculpting tools are much better and let you use alpha masks but they are slow compared to Mudbox and ZBrush, which use specialized drawing and sculpting engines. Where modo starts to chug sculpting on a few hundred-thousand polygons, Mudbox and ZBrush are able to push ten million polygons and still stay quite responsive. If you are doing a detailed face model, you are looking at a minimum of 3-million polygons, so keep that in mind. 3D-Coat is getting better by the version, but it still lacks high-quality sculpting tools so I rarely use it beyond voxel base-mesh sculpting. Don't get me wrong—you can get good results in some of the cheaper sculptors but if you want professional results, there really isn't anything that competes with the two expensive options...