1. Limit your connected devices
Limit the number of devices connected to your Wi-Fi. Now a days routers give option to even block or disconnect a user if you feel that it is consuming your more bandwidth.
2. Create Guest Users
If someone is visiting your home, do not share your primary password. You can create a guest network on your router with separate password.
3. Keep your Wi-Fi Router at center of your home
Usually people keep router at corner of the houses, to minimize wire clutter. But it limits your Wi-Fi range. The best place to keep it in center of house as its signal spread Omni-directionally.
4. Keep it at eye level or higher and not close to cordless phones, ovens, printers
Another way to have good signal strength is to keep it at higher position and away from electronic devices.
5. Change your password frequently
It might be possible your password is too easy to guess and can be used by your neighbors or surrounding people. As a result it lows your Wi-Fi speed and also monthly limits will cross early. So, keep changing your password with a strong password.