Most has already been said, but I do use a chamois and always have done for over 30 years of cleaning cars! So rather than repeat the good suggestions here (like two buckets etc), I'll add some of my own....
NEVER, EVER wash in direct sunlight or when the bodywork is hot to touch
Don't be tempted to add too much shampoo to the water just to make it more foamy - it just ends up leaving streaks on the car.
When you dry the car, try to dry in one direction if using a chamois or similar (rather than round in circles) - or pat it dry if you're using a big microfibre type towel to dry.
Once fully dry, an extra five/ten minutes work with a quick detailer spray (like Autoglym rapid detailer for example) makes a massive difference to the final appearance in my opinion.
I've also got a California Car Duster that I use to go over the car to prevent dust build up between washes - if you choose to use one, just remember to user barely any pressure and just let it glide over the paintwork and you won't do any damage (in my experience)
Black cars look amazing when clean, and when polished properly - but yes they are very high maintenance
I've not had my Leon a month yet, but have already clayed it, and given it a coat of Autoglym Super Resin Polish and Extra Gloss protection. It's already looking better than it was when I bought it from the dealer......
(not the best pic location I know, but it does show the shine I've got on it so far

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