While attending college, how can I best prepare myself for getting a flight school slot?

I will give you the same advice I gave to my sailors that were thinking about another path: Make yourself the most attractive candidate for whatever path you want. By that I mean:
1) Take and excel at any and all tests/qualifications that are needed
2) Seek out, and excel, at any extra military duties that will set you apart
3) In school, get the best grades that you can
4) Find any other extracurricular activities that will enhance your experience (again take leadership roles and excel at them).

My point with this is that it is much easier to get what you want, when you are the top 5%. Do whatever you have to get yourself there, but always have a backup plan or two. Remember that we serve the needs of the service.

I of course learned this all the hard way... I was at the bottom of my class at school, so there was only one thing open to me. Had a blast, anyways!

Honestly, it mostly depends on your ASTB score. Study the naval service stuff so you can get a great OAR on the exam for OCS, and get some flight time, or get yourself a rig for a flight simulator. The new ASTB is heavily focused on hand-eye coordination and multitasking, so short of practicing there's not much you can do there. Study up all the basics of flying; if you get accepted you don't want to be one of the API attrites.

Now before I start... There may be some minor nuances between the navy and marine aviation application process so take this with a grain of salt. I'm in close with a marine officer recruiter so if you want more details let me know. That being said here's my one cent.

Asvab: they don't care
Astb: the most important factor. Really interesting test. Familiarize yourself with basic flight-y stuff (maybe buy an astb prep book). Most of the test you can't prepare for though.
Age: for flight you need to be 27 latest by the time you are commissioned but a. That is extended if you have active duty time. I believe an extra year for every year you were active. Finish your degree asap. And b. From what the marine recruiter told me... Blow your pft out of the water and the age waiver will write itself
GPA: possibly the second most important factor. I believe it's a 2.5 minimum but at least in the Navy 3.2 minimum to be competitive.
Degree: not a deal breaker. Mine is in forensic psychology and from what I hear the navy and air force care more about the stem degrees than the army and marines do but I can't help but think a stem degree will help you at flight school
Pt: talking to the marine recruiter this as also huge. 280s or higher and you're cooking. The navy literally could care less which sucked for me because that was my main selling point.

In regards to your chances of getting it through OCS you have a great chance because the marines are one of the few branches that can guarantee you marine ocs air before you even sign the dotted line.

Anymore questions shoot