Write down common formulas that you want to learn for area and perimeter.
Turn the formula into a set of letters and words that makes sense to you. For example, you might express the formula for area of a triangle as "half of base times height" or "B H over 2." Express the perimeter of a circle as "pi D" or "radius doubled times pi."
Turn each word or letter in the formula into a cute character, accessory or action to make a funny story or image out of the formula. For example, "B H over 2" could become "Bunny Hops over Tuna" or "Bunny with a Hat over Two paws."
Modify a word or image in your cute mnemonic to help it remind you which formula the story goes with. For example, to remember that "Bunny Hops over Tuna" represents the formula for area of a triangle, you could picture the bunny hopping over tuna laid out in a triangular area, or picture the bunny's hop over a tuna as a triangle in the air to mimic the sound of "area."
Create distinct mnemonics for each formula you have to learn, then practice them over and over until you can successfully recite and use the formula every time without looking it up.