Other than obtaining a scientific foundation, as with A&P, Pathophys, Psychology, Sociology, Algebra, Chemistry, Physics, etc..., there is no sound method of getting a "head start" on paramedic school. Reading through the textbook before starting the class is useless. Learning IVs or EKGs before getting a foudation is useless, and as already pointed out, often counterproductive. The college courses are your best bet.
As for self-directed study, there are only two things that I recommend. First, get a self-study Medical Terminology workbook. Devote an hour or two a day to working through it. When you're done, go back and do it again. It will help you more than any other thing you can do by yourself. Second, get the Anatomy coloring book and a box of colored pencils and work through it. All of it. Don't just skip around to the pictures you think are cool. Work through ALL of it. You'll start class knowing more about anatomy than anyone in the class who hasn't taken A&P.
The Dubin EKG book is excellent, and you should definitely purchase it and work through it just before your class starts the cardiology segment. It is not particularly advisable to do that before the class itself starts though. Resist the temptation to focus on "cool" stuff like EKGs. You'll learn those when it is time. Until then, work on establishing a foundation.