When starting out, use a conversion sheet to give you a bit of a reference point regarding the metric system. This should only be used when you start to learn, so you can get a rough idea of how big or small, how heavy or light, how distant or close, or how hot or cold metric measurements are. For example, you can compare the current temperature in Florida to the current temperature in Alaska first using the standard system, and then using the metric system. So if Florida is 80 degrees F, it is approximately 27 degrees C. If Alaska is 20 degrees F, it is approximately -7 degrees C. Using this method, you now know that 27 degrees C is rather warm and -7 degrees C is quite cold. You can create a chart like this for yourself using length, mass, distance, volume and other forms of measurement.
Buy a scale that measures not in pounds, but kilograms; buy a meter stick instead of a yard stick; buy a thermometer that measures Celcius, rather than Fahrenheit. Stop using devices that exclusively measure in standard units. If you live in America, you may not be able to completely escape using standard measurements--for example, all distances are measured in miles, and gasoline is measured in gallons, not liters--but you can make at least some small changes around your home or office to help you better remember to the metric system.
Disassociate yourself from the standard system of measurement. It will be much harder to remember the metric system if you are converting every measurement from standard to metric. You must behave and think as if you were a person who had never before heard of or used the standard system. Your brain must begin to make new associations with metric measurements. For example, if you measure your height and find that you are 1.5 meters tall, it does not matter how tall you are in standard units. You now know how tall 1.5 meters is, as well as that everyone below 1.5 meters is shorter than you, and everyone above 1.5 meters is taller. If you continue to use the metric system exclusively, you will begin to understand how quantities and measurements relate to metric units. If your brain is constantly viewing the world in metric units, remembering the metric system will not be that difficult.