The only trick I know is to not blindly send out stuff without at least a small chance of a return on your time investment. Time = Money.
Like I said earlier, proposals are covered by a marketing budget. You have to assess your chances before spending the hours and you have to win a certain number of those jobs - or reconsider your approach.
For instance, if doing a design-level proposal, that would come after only a few companies were selected through a request for qualifications from the client. They review qualifications, and decide which three to five applicants to ask for a proposal. Sometimes, if pre-design is involved, they will pay ALL of the participants a stipend for that work. But only one company will get the job. Another way is to have yourself pre-qualified on various organizational websites where clients go to find designers, but for us, that’s mostly government stuff. I don’t know any such site for freelancers.
But, and this is a really large consideration, I work for a company with over 30 years of reputation and client-facing metrics to stand behind. I’m not a single freelancer looking for 'first work." Your location may matter too. I’m only familiar with USA circumstances. Things may be different where you are.
I suppose what you are talking about here is more a Request for Qualifications. Basically a resume and portfolio, with sound reasoning on why you will make money for the corporation where you are applying. Remember, the job isn’t about what you will learn or what they can teach you, it’s all about what you can do to improve their profit margin. A resume that starts off with, “I’m seeking a position where I can learn and grow” just tells me you haven’t thought hard enough about this job. I don’t want you learning and growing, I want you working to the best of your ability, and evolving to meet each new challenge, which brings in more clients and more money. Sounds harsh, but it’s business, not art class.