one source or sub-topic per card
record your research question on each card to help you stay focused
use one side, only, so that you can spread them out and organize them by sub-topic
make one card your Works Cited or Resources card and number each source: number items on cards to identify the source, especially if each card is a topic rather than a single source. NOTE: this is just an organizational strategy while you are working: you will need to properly cite according to the citation style (MLA, APA, Chicago, etc) and format the list of resources (Works Cited, References, etc) accordingly.
Here's an example of how you might use files cards--either hard-copy or virtual. This completely fictitious research project is about workplace stress. First you will see the note-cards where information is gathered around sub-topics that kept coming up in the researcher's sources. Each quotation is numbered. The page number from the source is in brackets at the end of the quotation, and then the source itself is identified by a number preceded by two dashes. All sources are listed on a Works Cited card. Finally the paragraph built using the information for the first topic is at the end.
Card 2
Card 3