While my petro collection is pretty pitiful (my money goes to expensive paints), I am very visual and I think these are general tips that could be helpful:
Think about your target audience. If it includes the general public, keep it simple. Anything technical may stop them from reading the whole card because they'll think it isn't for them. If there is more than one target audience, does it make sense to have more than one type of card?
In addition to handing them out, are the cards going in a holder for a show? Are you leaving them on someone's doorstop. ETC.
Must haves are of course contact information, but also a website (or as Nucky did a blog) address. You can't cram all your best visual stuff on the card, so the card becomes the opener for your website of wares or services. Almost everyone, even seniors, go on the net, tho I would also have a nice slim notebook sized portfolio with decent sized photos on hand when handing out your cards, because there are a few old timers who do not do the internet.
Do you want the back of the card blank so you can write additional infomation on it? Like the time you will be meeting them at their home, etc. They are more likely to loose that scrap of paper you wrote the info on than your business card, or will look at that scrap and wonder who they were to meet. One-sided cards limit how much you can put on the front.
Definitly a simple graphic that sums up what you do, with clear contact information as Wes and James have. Though I must say Nucky was able to combine an over-the-top eye catching graphic on a one-sided card with text.
If the target audience is over 40, and it probably will be, make the text bigger if you have the space. I hate getting out a magnifying glass to see someone's phone number.
Also if you do multi colors, make sure the text stands out and is easy to read. Don't assume that red will stand out on green if, for example they are the same value (same darkness or lightness). If they were different values, they would stand out. Do some mock cards and have older friends and family tell you which is easiest to read. A professional printer should also be able to tell you which color format, plus what fonts you choose are the easiest read. I'm guessing most printers will tell you not to go too crazy with colors and fonts.
If you have text on both sides, you can add more stuff to it with graphics with contact info on one side and the second side only text. Old Racer's a great example of that. You can't mistake from his graphic that this is a serious collector of lots of kinds of petro stuff. And the text clearly shows what he is looking for. His card would also do well as a table top hand out, or perhaps in his case, something to leave on the door stop.
Sorry to run on, but I thought these tips might help.