If you want to write an effective letter of recommendation, then try to follow these tips:

1. Never give exaggerated or superfluous statements as such letters very clearly stink of favoritism, bias and unnecessary support;

2. Do not try to be a very prolific writer of recommendation letters as such a reputation may make your recommendation a  piece of junk in the eyes of all those who have read or seen your "generosity" towards acquaintances;

3. Never use the same generalized sentences about the character, honesty, hard work, competence etc. of the candidate as used in every other letter which nobody likes to read;

4. Always try to concentrate or highlight the specifics or peculiarities of the recommended person as emphasizing on his professional skills. For example, "Mr Peter has a special knack of maintaining the computerized record of manuscripts" or "Ms Kelly knows her job well when it comes to cooking Chinese Cuisine";

5. Make an attempt to be honest and genuine while praising a person as it may also involve your credibility if the praised person fails to deliver contrary to whatever you said in the letter;

6. There is no harm if you slightly point out any small humanly possible weakness of the candidate along with mentioning some of his important merit or skill. This way of writing not only gives a genuine look to your recommendation but also makes the cited skill look more conspicuous. For Example, "Ms Lisa may be not as fluent in conversing on telephone as excellent she is in drafting and typing business letters";

7. Never say or commit anything for which you can be legally liable or anything that could cause any financial or physical harm to the concerned organization or individual. For example, if you have some feeling about the financial integrity of a colleague in that case you should never recommend his name for a job in a bank under any condition. Similarly, a chicken-hearted person may never be recommended for the job of a security guard.