Model your scenes out of scale. Make sure you don't have 1 km window or 5 mm car.
Set your emitter watts/efficacy, or lumens to insane values. Try to keep them fairly close to real world values. If the render is still too dark, change the Camera exposure values instead, just as you would in the real world.
Set unrealistic Camera parameters. Keep them in reasonable ranges just like in a real world Camera. Otherwise, you'd also set unrealistic emitter values to match the desired exposure.
Use full white (255,255,255) value or textures in Reflectance, especialy for wall materials. Doing so can only amplify the bounces, add noise and damage contast. A maximum of 240 is suggested for the most white surfaces.
Use maximum Roughness of 100 (Lambertian). Keep it between 95-97 for a more realistic shading on your roughest surfaces.
Use many BSDF or Layers etc for fun. Keep your materials as simple as possible. Most of the materials will not need more than 2 or 3 BSDF.
Use Opacity Maps (or Clip Maps) on hundreds/thousands of instanced leaves. Geometry leaves will be processed much faster than Opacity Maps.
Use bump/normal maps on everything, especially on distant objects. They are OK for close-up renders but keep in mind that very fine bumps have a negative impact on render time.
Intersect an emitter material with a subsurface scattering material or dielectric (glass, water etc).
Use emitters with big power differences between them. This will cause some optimisations in Maxwell to not work as well, and make the render converge slower. If you need very powerful and very weak illuminants in a scene, set their initial powers relatively close and adjust them later using Multilight in the post.
Use base material components (BSDF, Coating and Layers) for emitters without purpose. The base material will not be visible unless you turn off your emitter with Multilight.
Use Dispersion or Complex IOR unless you are making jewelry renders or products viz. Dispersion and Complex IOR are very costy effects for everyday scenes.
Use Coatings for creating everday clearcoats. Plastic approach using 2 BSDF (normal or additive) will be much faster instead.
Activate rendering of unneeded channels as each of them will add to the render time.
Use Multilight if you are fine with the emitters and will not make changes anymore.
Waste resources freely. Keep in mind, every subdivision, displacement, grass fin, texture, # of ml emitters are eating up your limited memory leading to a crash or serious loss in performane when the physical memory is over.
DO
Use simple geometry as your emitter surfaces. A simple single sided polygon will in most cases work very well. If you need a spherical emitter do use a sphere, but try not to make it 100 000 polygons....This won't have a hugely negative impact on the render time, but it won't help either. Plus your MXS scene file will be unnecessarily large.
Hide unnecessary emitters in the scene. e.g. Rendering an apartment interior with lots of room may have all the lights active and you can speed up rendering by hidding the rest.
Hide unnecessary objects in the scene. If the object is not directly or indirectly seen by the camera or having any influence in the scene, it's better to hide it. A porcelain cup in a closed cupboard with displacement could easily kill your benchmark.
Create new materials using Material Types unless you are not familiar with Custom material settings. Especially the Translucent Type is very helpful for avoiding the unwanted noise in translucent materials.
Prefer low poly geometry for shaped emitters instead of using Opacity Maps on emitters.
Use transmissive materials (glass, water, SSS) on watertight objects for avoiding artifacts and unwanted sparkles (namely fireflies).