Aside from your personal statement and college-specific supplements, teacher and guidance counselor recommendations are the next most significant pieces of writing included. This adds a sense of importance in picking the 'right' teachers to write your recommendations. The ultimate goal is that all writing in your application provides the most well-rounded view of who you are--not just as a student, but as a person.
Some factors to consider when choosing a recommendation include:
How well the teachers’ commentary on you will complement each other; i.e. if you have two math teachers, it might be best to do one and use your English teacher for the second recommendation.
What extracurricular activities you are involved in with them? If you have a club advisor, who is also a teacher you're close with, they might be a great candidate. They will comment on you not only as a student, but also as a person. They will know how you interact with your peers or deal with the stresses of a particular activity. This can be a great way to make your application more well-rounded and to include the most about yourself.
How well the teacher knows you beyond normal coursework? Have you gone to them for free periods? Engaged in intellectual discussions with them? Do they have a sense of your interests? The ultimate goal is to select teachers who know you beyond a letter grade. These are the people who can write you a unique, rather than generic, recommendation.
What will they say about you beyond just your grades in the class? Colleges already receive your GPA, and since teachers have various students scoring high marks, this alone won't add much to your application.
Keep in mind that you will, in most cases, need two long recommendations and 1 to 3 short recommendations. From your answers to the above questions, you should have a sufficient idea of who to pick for what. The teachers who know you the best and have the most to contribute should be given long recommendations.
Contrarily, those who only have a short bit to add should be asked for short recommendations. This is different at every high school, but at my school in particular, short recommendations were turned into our guidance counselors and then incorporated into guidance's recommendation. Make sure you are aware of how your school does recommendations, ideally by the middle of junior year. This way, you have the appropriate time to select and politely ask your teachers for the necessary recommendations.
Another point to acknowledge is that certain colleges have alternate recommendations types. For example, Dartmouth College has a peer recommendation that can be completed by anyone the applicant considers to be a peer. Do the proper research beforehand to know what the colleges you are applying to require of you to allow time for thoughtful contemplation beforehand.
A final piece of advice is to ask your teacher as early as you can (within moderation). Teachers are busy! They are going to have many students to write recommendations for and would probably appreciate knowing whether or not they have to write a recommendation for you in advance. This will help with their planning of your glowing recommendation too! Best of luck and happy planning!