I always carry cards around, and I give them to my clients liberally. Why? It's a sort of funny story, but it goes back to when I first became a PD and was so happy to have a real job that I could put "Attorney at Law" on my cards, that I got them right away. Many PDs I knew never got cards, saying that the last thing they needed was their names getting around the jail.
Well, I got cards, and when I first started out arraigning people ("these are the charges against you, you'll have a prelim/trial in so many days, what happened," etc..., but no real legal work) I would often have clients ask for a card. Knowing that I was not going to represent them any further, I still gave them cards. You wouldn't believe the buzz that went on in lockup after that happened. The client I gave the card to would go back into lockup and show it to someone else and say something to the effect of "look at what I got, my lawyer's a real lawyer, he gave me a card." The others would gather around, and then begin badgering me to represent them too.
Did it make a difference in my representation of them? You bet. As I have mentioned, people frequently (and incorrectly) think that us PDs are crappy lawyers, and that we're trying to dump them. Little things can dispel notions like this. One of those things are cards, it makes us look more professional. It may not help with every client, but it certainly helps a lot. Now, every time I meet a client for the first time, I give him or her a card, as if I was a high priced private lawyer meeting a well-paying client for the first time over a $300 dinner. It makes a huge difference, especially to someone who is suspicious that he is having some loser lawyer thrown at him for the sole purpose of getting him to plead guilty and go to jail for a long time.
The lesson: always look and act as professional as you can. It doesn't mean that you can't be a rebel (for many years I've worn an earring in Court), but it means that you exude confidence and competence, and people will believe it - even juries, all of which really matters.