Disclaimer: I have released precisely ZERO tricks through a major company and have only signed a single contract with a single company (unfortunately their manufacturer wasn't able to duplicate my creation on a production scale). There are people much MUCH more qualified to be starting this thread. Unfortunately, to the best of my knowledge, those who are super qualified to speak on this subject have not done so. Therefore, I believe some of these basics I've found to be objectively true could be very helpful to a new aspiring creator.

1. Upload your effect to YouTube separately for each company you plan on submitting to.

Most companies ask that you send them an unlisted YouTube video of your effect. I get the feeling that these companies are overwhelmed with endless garbage submissions and it is quite easy for your good submission to go unwatched and buried under hundreds of ACAAN variations and "unique" double lifts. By having a unique vid for each company, you know whether or not they have seen it. If a couple of weeks go by and your submission is still at zero views, you can pretty safely re-submit it without being a nuisance. If all of the companies have the same vid and it gets 17 views, you have no idea who's seen it and how many times.
It's a little tricky if it gets one view. That means their gatekeeper has seen it and has passed. I have moved on every time this has happened to me, but if you just KNOW that this trick is for this company, I guess you could consider redoing your vid and trying again.
If your vid gets 5 views or more, that means the gatekeeper has sent it to the team for a vote. You'll probably be hearing from them! If your vid gets 5 views or more and you don't hear from them, they have passed and the decision was made by a team. There is absolutely no reason to continue to bother them. Move on.
There was a certain effect I knew was perfect for a certain company. I sent it twice over a two month period and they didn't watch it once. The third time I sent it I had a contract in my email within an hour. I would have never known to keep sending it had I not had my submissions separated.

2. DO NOT, under any circumstances, bring up offers from other companies to increase your negotiation power. This is the magic industry equivalent of kicking a puppy.

I wish I could tell you "why" here, but all I can tell you is DON'T! It makes absolutely no sense to me and is an acceptable tactic in literally every other business I've been in. For whatever reason, doing this immediately puts you on their poop list and you have little chance of moving forward with them. Magic companies, explain yourselves!

3. Just about every company is aloof. Not hearing from them for a month does not mean they've lost interest. If they tell you they want it, be patient!

Your magic trick is the best thing ever in your mind, and when you're ignored for a month and a half it's hard to not feel a little insulted. You've got to remember, these guys have a bunch of stuff on their plate. If they tell you they want it, they'll get back to you eventually. You've just got to keep it in your head that no one else thinks your trick is as good as you do. I've made a huge mistake and maybe even burned my favorite bridge after moving on from a company that I thought had maliciously dismissed me for no reason at all. Learn from my mistake! (I've got to exclude Murphy's here. They've been prompt and attentive over every matter big and small. I think their massive team allows them to be so on the ball.)

4. Are you new and have a download you want to sell? My very simple advice is, don't.

I've only been in magic for a year and a month, but I've already handled cards for thousands of hours. I've got a deck in my hand nearly every waking moment. I think I've come up with some color changes, controls, and routines that are completely original and have value. I started showing them, and I was getting enthusiastic yes'! I was over the moon with excitement. Then in my research, I found that one of my color changes was not original, but a company was going to publish it anyways. I could've screwed up so badly here. Of course, this put doubt in my head about the other things I created. I ended up doing a little test and submitted something I knew was trash... and I still got a yes 😑 This was a little heart breaking. To put it bluntly, this is an area us new guys shouldn't get involved in. Maybe after we've been at it for five years we'll know whether or not we really have something.

5. Leave NOTHING to the imagination.

The company might tell you they can envision what you do when you explain the one part you haven't built yet or the thing you haven't quite got on camera yet... NO THEY CAN'T. They just can't. Don't even try. Get your effect 100% built and a vid that 100% shows it honestly. I should've put this first. Build the whole thing before submitting!

I really hope this helps some of you out. I wish someone would've told me this stuff before I started creating.