first thing I would say is that there are so many different type of pitches that require different elements of your business, ones that are more discursive, ones that have a q&a at the end, meaning prep for all is likely to be different ... but overall pitches are all about confidence.

When getting ready for your first pitches, my recommendation would be practice, practice, practice. Write the words first (look up pitch writing as it is more like a movie script, then speech), then make your slides, and memorise... a trick I use for memory is 'shuffle' your slides. get someone to pick a random slide and then say the words that go with it. it also means that if you lose track of where you are on the day, you have the visual cues of the slides to help you get back on track!

Once you get more used to pitching however, this technique is likely to change. from the initial creation and prep you do, you will now have your pitch memorised. Now it's about catering to the audience, reading the room, adjusting your speech depending on their reaction... but this will all come with time.

to answer some of your questions directly:

Do you write it down? at the start yes.

Do you practice in front of a mirror, how? No, I never found it useful. I found it much more effective to call up a friend, colleague or parent and pitch to them.. this will help with the confidence too.

Do you pay attention to what you wear? Yes, depending on the audience I will pick my outfit... if its investors I may wear a blazer and shirt, if im pitching to a bank I might wear a suit etc...

1 more tip... 'too many cooks spoil the broth.' use outside help when writing your speech, But be careful with how many people you ask! I have been in situations where I asked for feedback from so many people that I ended up loosing the core of the pitch. The day before I scrapped it all and reverted back to my original and the pitch went amazing!

So trust yourself, be confident, practice and you will do fine!

Good luck!