The military calls this "defense in depth". Rather than having a single silver-bullet solution, several different solutions are implemented at the same time, each contributing a part to a successful outcome.
To quote Wikipedia, "to be effective, control measures must be comprehensive, sustained, and systematic".
Use a flashlight, a mechanic's mirror, and a flushing agent (typically an aerosol containing pyrethrins). Inspect for signs of roach activity, such as roach droppings found where German cockroaches gather. Other signs include cockroach consumption of foods and the presence of cockroach egg cases and shed “skins”. These signs of infestation can help pinpoint where the roaches are living.
The use of pest monitors (“sticky traps”) also can reveal valuable information by helping to locate areas of roach activity. The use of monitors should continue even after cockroaches are believed to be eliminated. Leaving traps in place and checking them regularly can help confirm elimination, and give early warning of the presence of new roaches in time to control them before the population builds.
Cockroach hiding places often lie within a few feet of their food source. Look in areas that offer warmth, food, moisture and shelter. Remember roaches prefer natural surfaces such as wood and cardboard.
Younger nymphs typically do not venture more than 2 feet from their hiding places. Adults usually roam less than 10 feet from harborage in search of food, though a female carrying an ootheca may not move or feed until the egg case is detached. Obviously, knowing how the various stages of roaches in a population move is of great significance to any management plan, as are thorough inspections, good sanitation and exclusion, as well as the appropriate selection and use of pesticides.
Store food in insect-proof containers such as glass jars or re-sealable plastic containers. In severe situations, food trash can be stored in the refrigerator/freezer or in sealed trash cans [2], until it can be taken out.
Eliminate plumbing leaks and other sources of moisture. Increase ventilation where condensation is a problem. (more suggestions for reducing access to water)
(In pest-control terminology, harborage is any place that a pest might choose to live, sleep, and reproduce.)
During the day, cockroaches hide around water heaters, in cupboard cracks, stoves, crawl spaces, outdoor vegetation, and many other dark locations. They invade kitchens and other areas at night.
Remove trash, newspapers, magazines, piles of paper bags, rags, boxes, and other items that provide hiding places and harborage.
False-bottom cupboards, hollow walls, and similar areas are common cockroach refuges that should be properly sealed.
It's unknown how effective it is, but it might help help to caulk every crack, joint, electrical plate cover to reduce the number of places to hide. This might be especially useful in apartments, where roaches often travel from one unit to the next, through the walls.
Vacuuming is a highly effective method for reducing German cockroach numbers, as it physically removes the insects from the environment, and it pulls German cockroaches from their tiny crevices. Vacuuming not only immediately removes live adult German Roaches from the environment, it also removes dead roaches, roach eggs, roach nymphs, roach harborage, and debris that may become sources of roach food. In order to be effective, the vacuuming process has got to be entirely comprehensive. That usually means pulling out all appliances (refrigerator, stove, dishwasher, microwave), removing outlet switch plates, emptying kitchen cabinets, drawers, and pantries, and eliminating all clutter so that an vacuum extension wand with a crevice attachment tool can be used such things out of all tight spaces.
When vacuuming, be sure to use a vacuum with a disposable inner bag and discard the contents in a sealed container outside the home. If roaches are sucked into a vacuum cleaner bag, the bag should be immediately removed and placed out in the sun or in freezing cold conditions to ensure that the roaches are killed. Throwing the roach-infested bag into an outside dumpster is also an option.
Use non-repellent insecticides as the basic chemicals to properly deal with German cockroaches. Non-repellent insecticides are specifically designed to be transferred from one cockroach to another, which can effectively control the entire population. These types of insecticides are generally the better choice than repellent insecticides, as they don’t cause cockroaches to scatter and spread the infestation to other areas. However, rotating the mode of action of baits and pesticides is an important strategy to prevent the development of resistance in cockroach populations.
(TODO)
Advice for customers, for both before and after a professional treatment: