This seems to be the common topic of this forum, but I'll try to be more specific in my questioning. I am going to graduate from Vanderbilt this next spring with a major in Physics and a minor in Astronomy. My GPA will be average - about a 3.0 - and my major GPA is a couple of decimal points lower, around 2.7 or 2.8 (although my minor GPA is around 3.4 or 3.5).
I'm looking to go to graduate school, with the eventual goal of getting a Ph.D. I've talked to a professor of mine who works with a lot of graduate students, and he's said that my best bet of going to a good school (top 10ish) for a doctorate degree is to apply to a lower school for a Master's Program, do some good work, and then "slingshot" (to use his highly technical terminology) into a higher-ranked institution.
From what I've read, this seems to be the advice of most everyone here. However, I'm having trouble determining exactly which "slingshot" schools to apply to. Part of this is because I don't yet know where I want to end up, but hopefully that will materialize as I narrow my possibilities.
I've done some research, mostly around electrolysis using diamond cathodes. I plan to work with a new professor about the uses of graphene next semester, as well as with an astronomy teacher whose focus is on black holes and large-scale galactic formation. I think my eventual focus would be theoretical, or perhaps on renewable energy sources if I must pick something practical.

Any advice would be greatly appreciated. Thanks so much!