Give the students five symbols or so to learn a week. Once they have been introduced to all of them (just the ones you want them to learn), have them practice them and engage in friendly competition by playing Bingo.
Ex. for 8th grade: Have the students learn 35 elements (the more common ones). Give them a 5x7 blank chart. Call out each symbol one by one and let them fill in their Bingo card in whatever order they want. (Each student should be making them different so they're not all the same.) Next, actually play Bingo, and randomly draw little cards with each element name written on them out of a cup and call it out. The students will them have to find the corresponding symbol on their card. Once a student gets a whole row or column filled, they have to yell Bingo. They will then tell you all the symbols and corresponding element names for their winning row. If they are correct, give them some sort of prize, have all the kids clear their cards, and start over.
It is a good idea to make sure the kids do not clear their cards until someone is announced the winner. The person yelling Bingo can sometimes make mistakes, and then the round should continue.
Play as many rounds as you have time for. Have the kids keep their Bingo sheet in their binder so if you have a few free minutes during class you can have them play. Good, cheap markers for the cards are beans (give each kid a handful or in little cups).