Writing your press release will take time. Preparing mine took one month to get something decent to contact press. But, when it was ready, contacting press became easy.

Your press release will represent your game in the most synthetic way. That means you have to be sure of what your game will be. So sending press release very early in development can be risky because your game will change along its development.

Write a short press release. Press receive tons of mails everyday from developers. The more your press release is long, the more your press contact will ignore it. Less than 2000 characters is good but not more.

Know your contact. Show that you know the work of your contact and why you are contacting him. If you don't, why your contact should take time to look at your work? . To add a reference to an article related to your topic/interest from the press contacted is always a good thing.

Share a news that is worth it. There are a lot of things that can interest people to write about but there are more they don't care about. To help you find what you should share, ask yourself what makes you read the press.

Write an elevator pitch. It is the quickest speech you can tell to catch someone's interest while telling roughly what is your game. It have to lets people imagining what your game could be but not exactly what it is.

Look at other games press releases. The website Gamepress is a good place to look at other press releases and get some inspiration. You will see how successful games  talked to press.

Add only the most important links. Don't add to much links because your contact don't want to bother choosing which one explore. Best is to have 1 or 2 links max to give additional information but not more.

Tell to the press why you are contacting them.

An embargo is a way to tell to your press contact that your brilliant news shouldn't be shared before a specific date. Use embargo if it's vital but avoid it if you can. An embargo means 2 things:- it forces your contact to remember when to talk about your game, they hate it and they will forget most of the time.- it forces you to get back to your contact to give a reminder of your press release.

Don't spam your contact. : When other developers say "Spam the internet!", they mean "Share your new information everywhere once and do it again when you have a different/new one to share or other informations from someone else pushed away yours". Spam is a close repetition in time or space. Like repeating same thing in a small period or in the same place.