1. Collect what you like, and what appeals to you; don't chase "popular trends." YOU have to like what you're doing, not other people.
2. Always buy the very best you can afford; market tastes change, but quality never goes out of style.
However, my dad "stuck to his guns," in a gentle sort of way... teaching me about the inherent beauty of "high quality," and made "finding the very best" part of the fun of stamp collecting, even as early as when I was a teenager. By the time I was 12-13, I no longer had any interest in adding stamps with faults to my collection... unless they were high value classics I couldn't afford in any other condition. Even so, I was always on the lookout for "something better" to fill a space occupied by a ratty looking stamp.
40 years later, I am going about stamp collecting pretty much the same way, always focusing on finding "quality stamps" not just "stamps" for my various collections.
My dad knew what he was talking about. There's a lot of talk these days about the "decline" of the stamp market and our "dying" hobby, and how there are no new generations entering philately, and how many 16-year olds have never actually used a stamp on a letter... and so on. As I look around-- at large auction houses, or individual seller sites like eBay or BidStart or even the APS Sales Division, there's a huge volume of material for sale... suggesting there are many more sellers than buyers in the market.
Superior quality stamps-- old and newer alike-- are extremely difficult to find, and often command "stupid" prices, when they do show up for sale.
Of course, I don't actually collect "quality" stamps for their worth-- that's just a "coincidental incentive bonus." I collect quality stamps because the aesthetic beauty of something perfect or almost perfect appeals to me... and because I enjoy the "treasure hunt" challenge that goes with finding "the best stamp" rather than just "a stamp." This especially holds true for newer issues that are common and can be found by the thousands... yet finding a superb copy can be quite challenging.
Beauty, of course, is in the eye of the beholder. My dad originally wanted me to (or at least "hoped that I would") collect mint NH stamps because they were "pristine." It didn't really appeal to me, though. The number of ways we can mess up gum seemed "risky" to me-- all that worry about toning, foxing, wrinkling, fingerprints, moisture? No thanks! I also didn't like the fact that gum was one of the most forged/faked things on stamps... and it bugged me to have to pay so much attention to what was going on on the back side of the stamp FACING the album page, not the side you were looking at. But-- above all-- I liked the idea of having stamps that "told a story."
A used stamp with a legible postmark tells a better story, like "I was printed in Denmark in 1880. Somebody bought me, but didn't put me on a letter to be mailed from the town of Hillerød until 1898. Most likely I was on a letter next to another stamp, because the postage rate had changed from 8 øre to 10 øre between the time I was printed and the time I was used." To me, that's just more interesting. Romantic foolishness? Whatever...
Getting back to "beauty being in the eye of the beholder," beautiful postmarks are a major reason why I collect used stamps. Not only do I like the "story" aspect, and the challenge of finding that "perfect strike," but I like the additional fact that it keeps the hobby affordable for me... because I can collect a whole group of different socked-on-the-nose postmarks on the same inexpensive stamp, and still have them be "different," for the purposes of my collections. Postmarks can add almost infinite variety and possibilities to a stamp collection.
Sometimes I get the impression that quite a few collectors "lose interest" when they reach the point where filling "the next blank space" becomes incrementally more costly. Suddenly you find yourself with just 17 empty spaces in the album, facing the fact that the next space will cost you at least $100.00 to fill. It was a point I reached with my Denmark collection when I was about 16-17... and there was no way for me to continue the collection, on a schoolboy's budget... so I branched out.
But we all have different motivations for collecting stamps. It seems to me that the driving motivation of those who simply "collect one of each" is to have a "complete" collection. For me, the driving motivation behind stamp collecting is.... the actual collecting; the finding; the treasure hunt. In a sense, it parallels the popular saying that "life is not about the destination, it's about the journey." Having a "complete" collection just... just sounds too much like a "destination" or "goal," rather than an ongoing pursuit or hobby. I'm not looking for a point where I am "done" with my collection... I have no plans to reach "done."
That said, there is-- of course-- no "right" or "wrong" way to collect stamps.