In most cases, understanding and becoming familiar with a subject area is the best way to learn; however, sometimes we just need to memorize (i.e., what are the 6 types of veins?). Mnemonics are strategies that you can use to memorize difficult to remember lists, systems, and facts. Some examples of mnemonics are acronyms (HOME = Huron, Ontario, Michigan, Erie; MADD = Mother's Against Drunk Driving), first letter mnemonics (PIRATES = Pulmonary disease, Ischemia, Rheumatic heart disease, Anemia, Thyrotoxicosis, Ethanol, and Sepsis), or verses (Thirty days hath September,April, June, and November: All the rest have thirty-one, Except for February). You can make up your own mnemonics (you just have to make sure you remember what they mean!) or use mnemonics others have already created (like the examples above).
The wikipedia entry linked above (just click on "mnemonic") lists a number of different mnemonic techniques. I've also listed a number of other sites below that you might find useful.
Amanda's Mnemonic Page
University of Victoria's Mnemonics Page
Mnemonic Techniques provided by Inteligen