I highly recommend getting a referral from a friend or someone living in the same
condo (or wherever you are living). Also, the bulletin boards are great. I started out using 121 Personnel at Far East Shopping Centre, and they seemed okay, it was just that we didn't click with most of the candidates they sent to us (some fishy things in some of the resumes), or the couple of suitable helpers that we approached ended up taking jobs with other families already. One downside to interviewing agency provided helpers is that you don't get much time to interview. If you set up your own interviews with helpers who are recommended or posted an ad on a bulletin board, you can set up a much longer interview and get to know the person better.
I became aware of Singapore's only charity for helping abused workers because that is where our helper was staying. It is called HOME
http://www.home.org.sg/, and they have an agency called Starhome for helping to place workers who wish to remain in Singapore after their ordeals. The majority want to go home, but about 10-15% are actively looking for a new workplace. I recommend calling and asking about a transfer helper if you are truly a good employer. This means that you will provide a decent salary, decent work hours, private and comfortable place to sleep, work is in your home ONLY, every Sunday off as well as some public holidays, no verbal or physical abuse! HOME is also looking for donations and volunteers, so feel free to call if you are interested in helping this much needed charity in Singapore. Finally, please feel free to pass HOME's number to any helper you know is being abused or unfairly treated, because many helpers don't know that they have a place to go if they want to complain and take action about their mistreatment.