I receive hundreds of demos each month and 80% of what I see goes straight in the bin before I even hear anything. Here are 12 tips to help you not finish in the trash tab.
1 - Say hello and present yourself
Leaving a link and a "contact me if you like" is definitely not the way you should send your demo. Be polite and quickly present yourself, it is not necessary to be long, we don't have much time for this but we like to know a bit who you are.
2 - Respect the submission process the label wants
If the label asks for a private Soundcloud link, just send a private Soundcloud link. Do not attach your track to your email, just follow whatever the label guidelines are, you'll see it's usually not that complicated and pretty straightforward. 
3 - Do not send the same demo to multiple labels
Sending demos to multiple labels at the same time is one of the worst things to do. Imagine a label saying yes to your track after a week or two but you already signed the track with another one, this is a lack of respect. Most of respectable labels like to have exclusive requests. Start by doing your first choice label and wait to see what happens. If 2 weeks later you don't have any response then you can try with your second choice and so on. After 3 or 4 submissions without a positive response, it probably means that your work is not ready to be signed or maybe you simply contacted the wrong labels (e.g. your track is in the House genre and you contacted a Techno label). If you're sure about the quality of your work maybe you just have to start with small labels and grow your popularity from there.
4 - Look at what the label does before sending something
As mentioned previously if your track fits in the House genre and the label you submitted the track releases Techno exclusively, well, obviously nothing is gonna happen. Be smart and do a bit of research by looking and listening at the catalogue of each label you're willing to contact.
5 - Do not be pretentious, stay humble
Don't tell too much, the essential is enough.
6 - Do not send unmastered track(s)
We like to hear what the track is like. It needs to be ready to be played in front of the audience. An unmastered track is not representative of what the track will be at the end. Good things can happen as much as bad things depending on who does the master. Sending unmastered tracks to a label because you're already part of it, they know your mix abilities and they do all the masters themselves is another story. If you're reading those tips you're probably not at this stage so always send mastered tracks.
7 - Do not send a 2 or 3 minutes track
Ok we all know artists are releasing shorter music to break the Spotify algorithm. But if your purpose is to be played in clubs / festivals or to be in a Beatport top100, you have to send an extended version because that's what labels are interested in.
8 - Do not spam the label
Do not send 3 times your demos. Once is enough. If the label likes your track they'll contact you, but receiving three emails for the same track is pretty annoying, it feels like harassment.
9 - Have your socials up do date
We like to see who you are, how you promote your work, who is following you and how many people are interacting with you.
10 - Make sure your link is working
Double check your link, triple check if necessary, always. Receiving a link that does not work is annoying and many artistic director will pass on you.
11 - If the label is not from your country, speak English!
Do not speak your language if the label you contact is from another country. Just speak English. You really think we're gonna use google translate to understand you?
12 - Say thank you at the end of your message
Saying thank you, like saying hello is free and it shows you're polite which is the first and last thing we read from you. Really important.

In the next issue I'll give you some tips to be signed on major labels.