I would apply at the big departments you want to work at, and until you are hired, find a part time job at a small town. In my area, that would be anywhere from 600 to 5000 people, and every small town around is looking for some part time po-lice. Some of these places will even sponsor you in the academy if you agree to stay for a certain period of time, one year for example. After working at a place like this for a year, you'll be ready to rock and roll at a big department.
If your like i was when i started, you've never really even sat in a police car. I was twenty-one years old and working on a friend's farm making seven bucks an hour, which i was getting tired of. I saw the ad in the newspaper for poice academy and signed up. In Missouri, the academy cost $2500, and i paid for it myself because i wanted a j-o-b so bad. Luckily, i got a job at a pretty dang good dept two weeks after i got out and i've never looked back.
But, to answer your question, the best way to make yourself more marketable besides putting yourself through the academy is getting some part-time experience. You need to get used to driving a police car where everybody's watching you all the time, time to get used to wearing a uniform and a vest, and time to get used to taking calls and doing reports. I'm convinced that the best way to do these is experience-experience-experience.
Good Luck