In no particular order:

1 Patek Philippe are wonderful, but very often aesthetically boring. So, buy one only because you like it, but not for the name.
2 Never have a watch 'serviced' unless it stops telling good time. It's a waste of money and they'll only damage your watch.
3 Most watches made after 1970 are too expensive. So, consider pre-1970's watches.
4 IWC sold its soul to the Devil a long time ago, as did many of the great names, including Vacheron & Constantin and especially Omega.
5 Rolex is the only watch that people recognise on your wrist.
6 A watch that tells the time within a minute a day is accurate.
7 'nos' watches are often 'nos' because they have minor flaws.
8 Quartz watches are convenient.
9 Learn to accept damage to your watch. It happens.
10 Hirsch make good straps (and buckles).
11 There's no such thing as a black crocodile.
12 Secondhand Jaegers are under-valued.
13 The watch world is full of shysters.
14 Never buy a watch you haven't seen 'in the flesh'.
15 The price of the watch you buy is how much you can sell it for.
16 Never buy a watch as an investment. Buy it for pleasure.
17 Buy the most expensive watch you can't afford. It'll be good value later.
18 Watches are fun.
19 I have trouble counting beyond 12, which is why I don't wear a 24-hour watch.