1) Make the basics obvious
Your business card is a quick snapshot of who you are and how to get in contact with you. The content on the card should be enough to achieve just that without bogging down the small space available. Your card only needs your company name, your name, title, your direct phone line (not the main office unless they are the same thing), email, and website. Street addresses are not necessary unless you operate a local store that relies on in-person business.
2) Define your brand
If you haven't done so already, really take time to sit down and write down what you want people to associate with your brand. These can be images, feelings, words, etc. Once you have these, group them together and narrow them down to a few key points. This brand is what you want to achieve in a 2x3.5 space.
3) Make yourself memorable
Again, your business card is a small snapshot of you. It may or may not have a picture of you on it depending on the type, so it's necessary to make yourself memorable. A color scheme relating to your business or a graphic that brings it to mind with minimal effort. A good start is your company logo and/or tagline. Your business card is an extension of you, and therefore, your brand so be creative!
4) Keep it organized
White space is extremely important. An overly crowded card is distracting and not as aesthetically pleasing as a card that is organized and to the point. White space doesn't have to necessarily by white, but it does need to have space that isn't overwhelmed with content.
5) Make a couple versions
If you have several ideas, don't get too caught up on being perfect with the first card. Test out different designs, move around the information on the card, tweak the colors, and have fun! Try centering the most important graphic since that is where people's eyes first land. Or use different font sizes to attract from the complementary information in smaller fonts to the most important in larger fonts.