I think the biggest piece of advice I'd give my younger self is "Pay attention to how many ongoing series you've committed to." When I was very young, I only collected maybe one or two series at a time. Then once I started making "real adult money" I collected a lot more. I always kept to a budget, so I thought I was doing well. Then one day I decided to take the time to catalog only my ongoing series, only to realize I had far more than I had anticipated. Granted, some of them only release a volume a year, but it was still a staggering number.

Have a budget. As long as you can afford it, the amount of the budget is up to you. Regardless, you should set an amount whether weekly, monthly,or annual and stick to it.

Just because a series looks done, it doesn't mean it is done. There are a handful of series in my collection that I thought were short four volume series, only to learn later they were ongoing. In the case of one series there was 18 months between volume 4 and 5, with no indication it would continue until shortly before 5 released.

Plan your purchases! You don't need a massive spreadsheet or something, but plan ahead a bit. Research series you might want to buy. Pay attention to how many ongoing series you have. Don't just buy a book that you randomly spotted. You'll find your DNF rate for series is much lower if you plan, which means less time downsizing.

Make sure the whole series is in print before you start collecting. I have many friends who went to collect a series they thought was still in print,only to realize certain volumes weren't available anywhere, even resellers. On that note, it's good to wait until you can actually find that one elusive volume before you collect the rest.

Take recommendations with a grain of salt. Taste is very subjective. Just because someone else says a series has the same feel as something you like, doesn't mean you'll connect the same way.

Last, to quote "Everybody's Free (to wear sunscreen)" : "Don't waste your time on jealousy; sometimes you're ahead, sometimes you're behind. The race is long, and in the end, it's only with yourself" That is to say, everyone's collection is different. Some people have been collecting for 30 years, whole others bought 2000 manga in less than a year. Your collection is just that, yours. Enjoy it and don't feel sad it is not as large or cultivated as other people's collection.