A little disclaimer: All of these tips are based on my personal experience, what I've seen, what I've sent, and responses that I've both given and received. I've worked as a Marketing Coordinator where I've received thousands of PR emails and have gotten a sense of what works, what doesn't, and what you just shouldn't send.
What you should do:
Send a personalized email to any company you pitch: Always mention the name of the company, anyone you may have spoken to/worked with in the past, and address the company/person you are emailing. Anything else makes your email seem like spam and sounds like you were rushing.
Give at least one reason why you want to work with said company: If you are a fan of a certain product, campaign, or message, say so! The more details you can give as to why you're pitching a company, the better chance you have of your email being read, appreciated, and responded to.
Mention the specific product(s) that you're most interested in: This goes with the above -- Provide as many details and knowledge of the company as you can. If you're pitching a restaurant for a collaboration, mention your favorite dish or cocktail. Tell them about the time you visited with a friend and had a great experience -- Make the email yours and yours only. Add a personal touch.
If you've tried the company before, mention why you're a fan: A question I always ask as a Marketing Coordinator is "have you tried our products before"? Avoid the need to ask that question by mentioning if you have or have not tried a brand/company's products before. Throw in why you are a fan of the company and why you decided to reach out.
Link to your website or social channels: The less work the recipient has to do to look you up, the better. Link to your blog, YouTube page, Instagram, etc. so that the company can easily click your links, look you up, and see some of your sample work.
What not to do:
Copy and paste a generic pitch: This is not only annoying, but it shows that you're an amateur. If you're copying & pasting a pitch, chances are you are not addressing a specific company and are clearly in a rush. This shows that you don't truly care about your message and doesn't make for a quality, meaningful partnership.
Start the email with "Dear sir or madam": ...Just don't. Again, this makes you sound very spammy and like your email was hacked. Add a personalized touch: add the person's name, company, etc. Address them directly.
Say something along the lines of "I have X thousand followers and want to try your products": In case no one's told you lately, you are not defined by a number. Of course, numbers are important when a company is looking to develop partnerships/sponsored contracts, but show your personality, not your follower count. As mentioned above, just link to your social accounts so that the company can easily look you up and quickly see your follower count. You can throw a number into the email, but don't make it your headline or the main point of your pitch.
Send an email less than one paragraph: If your pitch is 1-2 sentences, it's obvious that you didn't write a quality email. Put some time and effort into it to get the results you're looking for!
Imply that you should have already received an outreach, sample, etc. from the company: This grinds my gears. Always speak in a positive manner and only speak of things that you can do in the future with the company. Mentioning that you "should have received or done X" only leaves a bad taste in the recipient's mouth. This makes you seem entitled, pompous, and simply isn't what one wants to hear from someone interested in collaborating.
Let me know your best tips for pitching a company, and also let me know your tips of what not to do!