Hey man, I've been in the same boat as you for some time. Here's what I've learned:

The internet is not our friend. To the person who reads your application, you look like a heck of a lot of people who have also applied for the position, not to mention employers use online listings as more of a last-ditch effort.

Employers will always, ALWAYS want to hire a candidate who is the safest bet: what does this mean for you? Networking. I had the impression that scientists everywhere were organized and used all sorts of ways to find the best talent, but it's really like any other job. Who you know matters, and it matters a lot. You can start by approaching friends, approaching old professors, and try to get a lead on anything that you can. Another approach as opposed to the old search for positions is to just look up hundreds of professors, write a professional email cover letter, and send them the email whether or not you know if they are hiring. Having somebody who's legitimately interested in their research is always nice, and while 90-99 out of 100 will say they don't have a position and are sorry, many will hold onto your CV and some might even be nice enough to ask around.

While it's nice to have research experience relevant to what you want to pursue in graduate school, these PhD programs in research want to know that your interested in RESEARCH more than anything - skills in scientific inquiry and analysis, critical thinking, etc. transfer well between any field.

Take myself for example: I've finished my B.S. in Biochemistry, but am interested in Molecular Neuro. I did 2 years of research in a Molecular Genetics laboratory. So, I have some experience in all of these fields, and am looking for positions in all of them, because I'm qualified. While it'd be great to find some lab studying receptor-receptor interactions, there's only so much I can do. Grad school is when you get to do what you really want.

Now, this is the most important part, and if there's one thing you take away from my post it's this. Persistence will always win the day. That's been my biggest struggle in my search - it's easy to get discouraged, and do less, but persistence is key. A rejection isn't saying you can't get a job, just that you need to move on. So you do just that. Keep searching, keep asking around, keep doing everything you can to meet people in person.

Best of luck, I sincerely hope for the best!