Mnemonics are a memory system or 'trick' that you use to remember a fact.
See Wikipedia: Mnemonic.
A large number of mnemonics exist for native-speaker school children to remember particular grammar facts. "I before E except after C" for example to help spell words correctly.
Link Words Edit
A more powerful vocabulary learning technique is mentioned by Wikipedia: Barry Farber in his book How to Learn any Language. He credits the technique to memory wizard Wikipedia: Harry Lorayne. The idea is to match the unfamiliar (that which is to be remembered) with the familiar (what which you already know). This is the basic principle behind all mnemonic systems. You use this for learning vocabulary by creating an association in your mind that makes it easy to remember. A couple of examples will make this clear.
An Example Edit
For example, lets say you wanted to remember the French word "chou", which means "cabbage" in English. "Chou" is pronounced like "shoe" in English, so you could imagine yourself putting on cabbages on your feet instead of shoes. Then, when you need to remember "cabbage", your brain will think "cabbage -> shoes -> chou". There is a vocabulary learning program called Linkword that contains mnemonics for a wide variety of languages. However, the mnemonics are generally more memorable if you come up with them yourself.
Here is an example for learning the Mandarin Chinese word "dùjià", which means "to go on vacation" in English:
“Dùde! Yeàh! We’re going on vacation!” – Hint: Say this one out loud in a “surfer” voice to get the correct pronunciation of “jià”. Also, while you say it, pump your hand dòwn twìce, like you’re using a slot machine.
Multiple Genders : Use Locations Edit
To deal with vocabulary for langauges that has multiple genders, Wikipedia: Dominic O'Brien in his book "How to Develop a Perfect Memory" suggests not only having a scene you remember, but locations as well. For example, using a familiar city as your "base location", you would divide up your city by a large north-south street. All locations on the west side of the city would be used for masculine words, and all locations on the east side would be used for feminine nouns. So, your mnemonic for "le jardin" would take place in a location on the west side of the city, but "la confiserie" would be on the other side. If your language has more genders, then pick another east-west street to divide the city up into 4 sections. For multiple foreign languages, you would use different cities to prevent then from getting confused in your brain.
Further Reading Edit
For more information on Memory systems, see the above listed books or check out The Memory Page.
There are more premade mnemonics listed at MandarinMnemonics.com.