Scooby,
In general, you should obtain one recommendation from a science teacher, humanities teacher, and extracurricular supervisor. In this way, you can prove your excellence in science, liberal arts, and non-academic endeavors. Its all about showing your well-roundedness. Focus on obtaining recommendations from individuals who know you well as opposed to big-name professors you have never met. A glowing recommendation from your advanced biology teaching assistant whose office hours you visited weekly will be much stronger than a two-line recommendation from your dads famous researcher friend who you met once at the mall. Coaches, community service leaders, and employers. and principal investigators can make excellent recommenders.
Some schools require two science recommendations. Check with each school for specific policies. Also, some schools do not count math as a science. The Texas schools can be particularly picky about such things. You may want to hunt down that old lab teacher even if he is no longer with the university. Also, you can ask former science TAs who may have known you well. The science rec doesn't have to be from a big name prof. If it comes down to it, perhaps you could schmooze that old pre-med advisor and become one of his favorites! I ended up obtaining a physics professor rec right at the end of the class and a biology TA rec. I also submitted one from my basketball coach, research advisor, and a humanities prof. It worked out well - I got into Harvard.
Let me know if you have any further questions.
MDAdmit