My own journey started in earnest in late 2008 - before that, I'd owned a few watches, but seldom really wore more than one for extended periods. In other words, before 2008, I think I was a pretty normal wearer of watches.
However, since then, my journey has been through many phases, and I have learned a few lessons. In the spirit of learning from others' mistakes, maybe it will help if I try to document mine:
1. More is not more.
For some of us, owning dozens of watches is a goal in and of itself. At one point, my collection reached around 40 watches. Here's a picture from 2010...

At some point, I realised that I wanted to wear my watches, rather than keep them in boxes, and that therefore, I simply had too many. Since most of them were mechanical, I also realised that the servicing costs would be quite severe. Consequently, I decided to aim for a smaller collection - and I have to say it definitely suits me better.
Pretty much all of my watches are now in my collection for a reason, and serve a specific purpose. Consequently, they get used - which for me at least, is the main point of owning them.
2. Do not buy solely on price.
I went through a phase of buying 'bargains', where a good price was the main driver. I don't mean that I bought only cheap watches, but rather that I was buying because of the 'deal', rather than because of the watch. This led me to buy some pretty unsuitable watches, which of course, did not stay long. In some cases I was able to recover my outlay on sale (even making small profits here and there), but I also took some losses. In any event, owning a watch that you would not have bought at normal price is not too satisfying IMO.
3. Watch out for the herd instinct.
When you are a frequent visitor to watch forums, it is all too easy to get caught up in the clamour for the latest forum darling. I have certainly been guilty of this on several occasions - sometimes it's fine (Tudor Black Bay), and other times, I have bought the wrong watch (Seiko Monster, Seiko Samurai).
4. Buy what you like.
Almost the opposite of the above. It's easy to be swayed by forum opinions, and sometimes, for some reason, the crowd dislikes a specific watch. In the end though, if you like it enough to buy it, buy it.
5. It's not for free.
Like all hobbies, collecting watches has a cost. Of course, careful buying can minimise risk of heavy losses, but (with a very few high-level exceptions) watches do not make good investments. So buy, own, enjoy, keep, service, sell, whatever. But expect it to cost something.
6. Accept no substitutes.
Of course, within reason. If you have set your heart on model x, with strap y and a specific caseback, try to avoid buying something "close" to your ideal. Near enough is rarely good enough, and you will probably regret it.
7. The thrill really is in the hunt.
Taking time to decide what you want, searching for the right example, the best deal etc, is all part of the satisfaction. Rushing into purchases can often prove unsatisfactory.
8. Judge watches in your hands, not online.
I often read posts saying "I like everything about watch x, except the... (insert tiny little detail)" or "I like all watches made by brand X". From my own experience, looking at watch pictures is all good and well, and can definitely provide some background knowledge, but there is no substitute for handling them, trying them - even living with one for a while - to really discover whether or not that "tiny detail" is important, or that in fact some watches in the range of Brand X are not as good as others.
9. Experiment.
Despite all of the above, there are situations where you need to step outside the framework and experiment. This where you're on your own. You need to be streetwise, and most of all, know your own mind. In some ways, it's a combination of all the rules because you may be playing the long game, or you may be impulsive. Yet it's against them all at the same time. It's down to your own risk appetite. What's pocket money for one may be a big investment for another. But the experiment can be either the acceptance that resale may at a loss, or it may be a rock solid investment such as a popular Steinhart or a Rolex. Either way, you should go into an experiment fully informed, and with an escape plan.
How else will you really know what you like if you don't experiment?
10. Always buy on the bracelet.
Bracelets are usually much more expensive to buy separately, so always consider buying the bracelet with the watch, even if it takes you over budget or you don't fancy it right now. Aftermarket straps are usually easy to match to watches with standard spring bars, however non-OEM bracelets rarely look the part.
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This list is not intended to be lecture, and for sure some of it will not suit you, but it took me a while to learn some of these lessons, so I thought it may be worth some consideration.
What other lessons are there?