Keeping an informal "grammar log" on each student can help you address their most glaring problems and keep you from allowing grammar to take over the focus of the class, which is, after all, about writing. When the student hands in his or her first paper, identify one or two repeated errors, and jot them down in a computer file or your gradebook. Explain, either on paper or in a conference, what the problem is and strategies for avoiding the error in the future. Give the student supplementary reading from the Prentice Hall Guide handbook section, and encourage him/her to focus on correcting that error for the next paper. It's important to note that most ESL students understand English grammar very well -- but not necessarily in the context of their own writing. Always keep grammar instruction focused on the students' own work, and that will help them learn to recognize similar problems in future papers.
Occasionally, it's helpful to cover broad concepts in class, such as punctuating quotations. Always refer the student to supplementary readings in the Prentice Hall Guide so they can look up any questions they might have later. Refer them to the Writing Center as well if they want extra help identifying and correcting their mechanical errors. Emphasize that the Writing Center does not provide proofreading services.
Many problems with syntax arise when students attempt to "translate" their work from another language. Encourage your students to write in English because translating is rarely effective. Have them read their work out loud also. Many students will "hear" syntactical problems before they will see it on the page.