Most scholarships, especially those for more than $1,000, are awarded by the college or university. Merit scholarships are now routinely awarded with the acceptance letter if it is a private school. Public universities often post their automatic merit scholarship standards online. GPA and SAT or ACT are the key factors in determining the amount of scholarships. So the advice to practice for those exams is great. As for GPA, if her high school offers both Honors and AP classes, tell her to not feel compelled to take AP classes in subjects for which she thinks the Honors classes would be better for her. (My daughter just went through the process last year and I have served on admissions committees as a faculty member at a university several years). If she is applying to smaller (<3000) private schools, showing interest in the school also can affect the award amount, so be sure to visit. I strongly disagree with the idea that the essay is the most important factor. Essays do not affect award amounts much for merit scholarships, but may have some small impact for diversity or specialized subject awards (such as a women going into engineering). Frankly, admissions officers and committees know that many applicants have parents, teachers or counselors edit the essays like crazy. I think they should have an hour at the end of the SAT/ACT for a student to write an essay to avoid this issue.
There is no point in applying for specific school scholarships in advance, but contacting or visiting schools would be good her junior year. If she is interested in a particular subject, she can look for writing competitions, etc, that offer scholarships as prizes.