One of the most popular techniques to remember lists is to use a mnemonic device, using the first letter of each planet as the first letter of each word in a sentence. For example, one sentence could be My Very Expensive Malamute Jumped Ship Up North (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune). Experts state that the sillier the sentence is, the easier it is to remember. You can make up different sentences to remember the size of the planets in order or even the dates they were discovered. Make sure that you do not come up with too many sentences to keep straight though.
Some people are visual learners, so creating a sentence does not work very well for them. If you are a visual learner, then you will have to find another memory technique that works for you. Try drawing a picture of the planets in order. You do not have to be an accomplished artist to do this; you can simply draw different circles for each planet. Use a specific color-coding method to help you learn quicker. For example, use red for Mars and blue for Neptune. Whatever you decide, try to pick colors that are radically different to avoid confusing them.
If you enjoy working with your hands, then building a three-dimensional model of the Solar System might be the right method for you. You can buy an inexpensive Styrofoam model at your local craft store, and it might be a fun project to do with your kids or alone. If you are looking for a group project to help a class of kids learn their planets, then you could arrange a contest to see who comes up with the silliest sentence to remember the planets. Additionally, you can have eight children act as the planets while the rest of the class tries to line them up in order. You can use these tricks as a starting point and find more ways of remembering the planets that work for you.
Astronomy Cast has an entire series of episodes on the planets. You can get started with Mercury.