May I suggest reading the articale on wetwebmedia.com
Generally, I have always found yellow tangs to be hardy and easy to care for - I have always had one for the last 10-12 years or so.
The usual things apply - get a good one, make sure he is q-tined properly, feed the correct diet and make sure he isn't bullied, and finally ensure that water quality is ok...... aside from that, I would consier them a beginners fish and very hardy - which suggests to me that either (a) your yellow tangs have been bullied to death, or (b) your source is supply you with bad fish.