Say only what you believe. Be aware of your reader; don’t talk down to her, and don’t yearn for love from him. Keep in mind what he or she knows already, admires already, might believe already; those baselines will differ in different venues (the readers of the NYTBR aren’t the same as the readers for N+1, who aren’t the same as the readers for Rain Taxi). Keep your eye on the work, which in poetry reviewing means the poems; do not tell me about the poet’s life or the book cover (mysteriously, beginning reviewers often describe book covers at length) unless those things become important in the poems.
Remember that even the dullest book was written by somebody who meant well, who wanted it to stand up, though (alas) it falls down; if you are going to say that it falls down, and there are circumstances under which you ought to say so, you should, try to say so without malice, and without glee.