Theme Hospital

The default "tiredness % before allowed to rest" setting is too high. If left as is, your doctors and nurses will eventually get mad and demand insane wages before resigning. Reduce it to 25-30 percent from the policy menu and they won't make a peep unless something is seriously wrong in your hospital (a vomit wave will demoralize the handymen cleaning it up for example).
In the beginning of a level, you can get a head-start on your competition and first dibs on the staff pool by pressing the "1" key to set the game speed to a minimum (you can use 1 TEMPORARY-DASH-CHARACTER 5 to set the speed, with 3 being the default).
While the game moves slowly plonk a reception desk & receptionist down. A single receptionist is enough in most levels. Then click on the hand above the timer to get your hospital going straight away. Continue in the slow speed till you have a GP office and you have hired every competent doctor possible.
Generally you always want the following staff members early on: two surgeons, one psychiatrist and at least one consultant doctor. New staff members become available at the first of every month.
Putting consultants in the GP's office will solve 90% of cases, or get you 90% of a full diagnosis. It's great for weeding out the cheap and easy diseases.
A single consultant researcher is enough to keep your research department on track outside of the finals levels of the game. If you don't have one you could use two lesser researchers until you get that consultant.
Research all the cure and diagnosis rooms first (F4 is a hotkey for the research screen).
Layout is very, very important. Clicking on the map button at the bottom of the screen shows other areas on the ground you can buy (as well as change heating level for radiators BTW) Look and plan areas to use for specific needs. Like one area just for diagnostic areas, another for GP offices, treatments, cures. Build your pharmacy close to your GP office. Build a ward next to operating theater. And don't make the smallest sized rooms possible, open them up a bit on each axis with windows everywhere as it makes doctors happier and always plonk an extinguisher, radiator, plant and bin in a room. Have 2-4 GP offices after level 3/4 and keep the area outside spacious but with a good amount of benches. This is the busiest area and it’s easy to get a vomiting epidemic going if it is messy or too cluttered for patients.
There are a lot of small things you can do to optimize the usage of a room. For example with a GP's office place the patient's seat next to the entrance so they just have to take a single step into the room. Place the filing cabinet next to the desk so the doctor need just two steps to go back and forth. Simply pay attention to how the staff and patients move around a room and move objects around if they take a lot of unnecessary steps.
Removing a room seems to refund most (if not all) the cost of building it. Don't be afraid to make use of the space you have until the moment you have a steady income and you can start expanding the hospital.
The effectiveness of the training room is divided between all students present. 1-on-1 instruction in a large room with lots of bookshelves is the fastest method. Eventually it's a good idea to have one wing of the hospital dedicated to the training and research rooms with their own staff area to rest up quickly.
Try to aim teaching one useless (=cheap) doctor in all the skills for being a psych/surgeon and then keep training him until he becomes a consultant. Then use that guy to teach other rubbish doctors. This allows you to have flexible consultants who cost less than 100 bucks whereas the consultants you hire directly will have a wage of 300+ bucks.
Avoid casualties; don't botch emergencies or guess with the diagnosis. If you go the whole year without any patient deaths, you get a fat $10,000 bonus. The game boos you when someone dies, find who it is quickly (usually the pharmacy) note the game date and maybe reload an earlier save (it does auto-save now and then) and send them home near that date.
Be cautious of emergencies where there are more than 6 people, it is generally very hard to cure that many in time. Especially if your helipad is a long way from the treatment room. Send a couple of the patients home if need be, you still get some of the cash reward for those that you DO cure. Just don't let patients die in your hospital!
If someone is dying in the hospital (sad face turning into skull icon above their head) and you can't cure them in time click on the door of wherever they are queuing, find them in the queue and left or right click on them and you can send them to another hospital! I do think it hurts your competition. If that’s difficult to understand just send them home.
You want staff out of the staff room quickly. Just build sofas, no pool tables or arcade machines. Check their morale regularly and if it is low give them a (one-time) bonus as their (permanent!) wage increases can be huge.
You can't do much about rats but placing a corridor item over the hole removes it for a while. If there is a hole under a corridor item already then pick it up and put it back to get the same result.
Earthquakes come up after level 6 or 7 so keep the machines repaired or they blow. Every time it is repaired it loses one off its maximum health so replace them now and then completely. A large, large earthquake takes about 5/6 units of health off all machines so watch out for the cure machines they generally start off with low maximum health. The research department gives them a better maximum health level but you need to replace the machine to get it.
Once an epidemic timer runs out the minister of health will come to visit your hospital. He will walk towards a reception desk, but not the closest one to the entrance, he will walk to the last reception desk you moved around. You can exploit this by building and staffing a second reception desk in a remote corner of your hospital. The extra walking distance will buy you time to cure the epidemic or even to send all the infected patients home.
NEVER grab a doctor when they are in the surgery room. If they are wearing their surgery scrubs and if you drop them outside of the room the game will start bugging out. At first this will just cause graphical bugs but eventually it will lead to the game freezing. If it does happen, reload an earlier save.
If you buy the game from GOG the default sound-card settings will give you very crappy MIDI music. Here is a link to a thread on the GOG community forums to improve the music: https://www.gog.com/forum/theme_hospital/fixing_the_dreadful_default_midi_rendition