There are arguably few things worse than the sinking feeling you get when you return home only to find that you have lost the key to your house. Even if this hasn't happened to you, you've probably at least found yourself in a rush to leave home but unable to find where you left your keys. Don't worry. If you keep a couple of simple things in mind, you can always keep track of your keys.
Consistency is the key to keeping your keys.
Decide which pocket you will carry your keys in and stick to it religiously. You should always know to check your left front pocket for the keys, for instance.
You need a safe place at home to keep your keys, and you must never put them down anywhere else. This means that you have to empty your pockets as soon as you get home.
When you use your keys, make a habit of carefully replacing them in their proper pocket, just as you make a habit of locking and unlocking doors. This will require a conscious effort until your routine is firmly established.
Make a habit of always checking your pocket for your keys before you leave a place. Just a quick pat of your pocket (and wallet!) will assure you that you have them.
An alternative is to choose one place in every room where you can put down your keys. That way, in case you are not able to put your keys in their normal place in the house, there are much fewer places to look before you find them. You just have to look in one spot in each room to see if your keys are in that room.
If you don't wear pants with trustworthy pockets, you will have to carry your keys in a jacket or a pocket in your bag. The same principle of consistency still applies.
If all else fails, make sure a neighbor you trust has your house key, or you can hide it somewhere only you will be able to find, e.g. a flowerpot.
A cheap rock climbing carbiner with a key ring clips nicely to a belt loop if extra security is wanted.
Beware of staking a claim to a "key spot" in your home without consulting with your housemates. You don't want someone to "clean up" your carefully placed keys.
Once you've established your routine, all you'll have to worry about are the times when you break your routine, such as when you wear shorts. These times call for extra attention to your keys.
Never put your keys under something that looks like it could be safely stepped on, like clothing on the floor. You could break your keys, or hurt yourself.