If you are trying to cut your SIM card down to a MicroSim, forget the knives or scissors and just order this MicroSIM Cutter. The product is on sale for $12.47 on amazon.
MicroSIM is use in iPhone 4 and iPad 3G, a smaller version of a standard GSM SIM card. DIY SIM to Micro-SIM conversion is free, but harder than it looks. However, if you cut it wrong, you have to go to the wireless store and buy another sim card. So, you better watch out your hand.
Well if you want to minimize the risk breaking a perfectly good sim card, then this is for you. Micro SIM Cutter works similar to a hole puncher. Is very easy to use, you simple need to put your sim card in, press down, and get a pretty good cut.
Video: MicroSim Cutter