If you don’t want to use the little “bell ringer” you have choices:
a)  keep the front door locked and ask people to ring the doorbell.  Then look to see who’s there & buzz them in. That’s a good way to go, a little more expensive, but it works.  It also strikes a number of people as “unfriendly”.  On the other hand, if your client base is abused women, they might like the idea.  It depends on the practice area.  You will have to decide for yourself.  It’s a good system to use for weekends and for when you and your staff are working late.

b) a pressure plate under the rug where people step on it as they come in and a remote bell rings/buzzes.  Unnoticeable.  Also not too expensive. I think it’s a little less than the buzzer system but you will have to price it out.

Most of these things can be gotten at Radio Shack for not much and are easy to install. Have fun.