(By the way, this list is a living document: if you've got an idea what to add or change, please let me know.)
▪ Make a plan in advanceJust improvising with a couple of parts and hoping it might show the outline of something interesting can be great for collecting ideas and inspiration, but will rarely lead to a really great model. Good building should begin long before you've picked up the first brick ― by then you should have a clear idea what is the thing you want to build, the rough proportions it is going to have, and a sketch (at least mental) of how will the various critical components be implemented. Otherwise you might often find yourself in dead ends, when you need to change two thirds of the existing structure to implement what you've wanted. Or even worse, discover that the whole model is to small to allow the desired functionality.
▪ Reinforce the structure, though not excessively
The need for reinforcing beams is pretty obvious when building Technic stuff, and any construction should therefore have at least some sort of a sturdy skeleton (don't rely too much on the friction between the parts). However, motto "the more, the better" does not apply. Excessive reinforcements increase weight, which in turn increases the complexity, requiring even more reinforcements, etc., and makes the model difficult to disassemble or modify. Use a few cleverly placed beams where they do not disturb the function, yet attach to as many "loose" parts as possible. Triangular structures, usually done by attaching diagonal beams across rectangular structures, work nicely.
▪ Avoid excessive speeds, forces and gearing ratios
Moving parts have a limited range of efficient operation. Try using gearboxes and levers where possible to keep everything within that range. Otherwise, if you let the axles or gears spin too quickly (over cca. 1000 rpm), the friction will noticeably degrade the available torque. On the other hand, if you gear the rotations down too much, the large resulting torque (say, over 0.5 Nm) will put unnecessary strain on the axle and its supporting structure, as well as the components it is attached to (small gears are quite vulnerable).
▪ Minimize the number of gears, and consider protection from breaks and blocks
Try to achieve your work using the least amount of transmission components possible. Each gear increases friction, and each gear pair introduces backlash in the transmission, while the total backlash increases with each pair. It may be a problem if your model requires accurate movements, e.g. if you are controlling small movements via NXT motors ― in those cases, consider the gearless design, i.e. attaching the target axle directly to the motor, or keeping the whole system under light tension with springs or rubber bands. The other component always worth considering in transmission systems are belt drives (also using rubber bands) and clutches ― they will protect the parts in case something gets blocked. However, if facing considerable torque, they will reduce accuracy, so these two requirements need to be balanced wisely.
▪ Keep practicality in mindIf a model is large or cumbersome, make sure that it has at least a couple of easily reachable "strong points" used to carry the model in hand(s) and perhaps drive it around wihout fear of damage. Of course, not every model needs them: some, such as cranes or off-road cars, are usually so heavily reinforced by design, that almost every beam in their chassis is a strong point anyway. Keep the controls easily reachable, and unless they serve as basic structural parts of the model, allow the battery packs to be removed and reattached (or, recharged) easily in case they run flat.
▪ Use special parts for special purposesIn addition to the previous rules: when building a model where plenty of force is necessary for its purpose (cranes, lifting mechanisms, superstructure movement), use the parts Lego dedicated for the purpose. For example, use a compact frame where you need sturdiness, instead of building one with beams. Do not try rotating a large superstructure on a simple axle; use the turntable instead. Where you need lots of push or tension power, use pneumatics or linear actuators rather than control arms. It is always a good idea to apply these "power parts" as late as possible in the system process, to reduce the number of bricks exposed to their force. Keep in mind that the power stage usually needs more reinforcements, too.
▪ Prefer modular constructionIf you can split the planned model into several separate components that can be simply assembled in the final phase, you will find the model easier to build, modify and optimize. For example, chassis and superstructure of machinery; chassis, suspension, transmission and the engine in cars; wings, fuselage and landing gear in aeroplanes; etc. However, keep in mind that the modular approach isn't always feasible: certain models, such as walking robots or supercars, require too much "integration" and coordinated cooperation of all components, and would therefore be more complicated to build in modules than in a single construction.
▪ Keep the weight distribution in mindDespite being difficult to predict accurately until the construction is done, it is wise to at least have a general idea of the centre(s) of mass and where they should be, and design the construction accordingly. Many models, such as cars with suspension, cranes, walking robots, manipulator arms, etc., will work much better (or only) if properly balanced. It is a good idea, if the construction allows, to leave some extra space available for the heavy parts (such as motors, smart bricks, etc.) that are placed at the very end. That way you can easily fine-tune the overall weight distribution by repositioning the heavy parts.
▪ Decorate through small details and integration with the skeletonTechnic models usually don't need too much decoration. Small details, such as lights, antennas, exhaust pipes, etc. work nicely. Other good idea is to use reinforcement beams as a visual element ― usually outlining the desired contours of a bodywork, which is a common trick among official Technic models. Just make sure that decorations never obscure the underlying mechanics completely, or even worse, make the controls harder to reach or notice.
▪ Document your workFinally, it is a good idea to document your models, or at least their particularly clever systems. A sketch will work nicely, photograph as well, and a virtual 3D model even better if you've got enough time and patience. Unless you have perfect memory, it will be useful for future reference and inspiration when you face a problem you have already solved well in some of the earlier models, and for communication with fellow Technicians.