Just writing a recommendation about a person isn't enough to persuade someone that he should believe what you have to say. Specific information about how you know the applicant and why the applicant is a good fit for the position is critical for the recommendation to work. Think of the specific reasons why the person asking for the recommendation is the person for the job. Tell the reader how you know the applicant and why he is worthy of your recommendation.
Try to match the applicant's specific qualifications to the position or scholarship that he is applying for. Using your knowledge of the applicant, form the recommendation to specifically highlight the reasons this particular person is qualified for this particular job or award. It helps if you have a good knowledge of the applicant beyond just a passing acquaintance. Generalizations about the candidate's suitability usually are not very helpful in the recommendation process and are to be avoided.
Letters of recommendation that go from one commendation to another are generally not considered a strong letter of recommendation. Praise should certainly be given, but it must be backed up with concrete reasons for the writer's regard. The applicant should have done something worthy of the praise, and it should be clearly documented in the letter of recommendation in order to be helpful to the deciding committee. Flowery praise that stems from nothing more than hyperbole is just wasted space in a letter of recommendation.
Giving criticism in a letter of recommendation is not to be done. It reflects poorly on the applicant and can seriously damage an applicant's chances of success. If you cannot give an honest and positive letter of recommendation, it is better to tell the applicant "no" than to write a letter that is critical. Honesty is to be expected from any letter writer, but criticism and backhanded compliments will only hurt the student who put his trust in you. It is more professional and better for the applicant to decline writing the letter entirely.