Ask questions and learn from expert freelancers
You'll need to be an expert in the field, able to summarize your experience and abilities in a few characters. Employers can tell if you're simply fishing for jobs without the expertise to handle the work.
I also like to add something a little personal, which serves as proof of my experience and my genuine connection to the work. For example, I love the performing arts, and I said so when I was applying for art-related jobs. Now, my work is as a freelance writer; there's nothing "performing" about it! However, the first job I was hired for was a performing artist who needed help with some writing. S/he knew that I have a passion for his work and would be dedicated to the job.
So, let's say that you specialize in cleaning and analyzing data. Pay attention to the organizations you're applying to and what the data set is. Maybe one of the jobs is related to a water-treatment center, and you happened to really love biology in school! Tell them that. The extra information tells then (1) that you have a slight edge in knowledge over other bidders, and (2) that you have a personal investment in the project.
These are my thoughts, and I hope they might be helpful to you! I summarize it up as this: employers can tell when we want the money but we don't care about the work itself. Show them that you love the work!