Build a solid knowledge base about the subject of your collection
If you follow the other three principles, this one should just naturally develop. It's one of the primary reasons to collect something you're really interested in, instead of something you hope to make a quick buck on. Because research is an important part of collecting, and having to study a subject you're especially fond of is, well, too much like work.
So where does this knowledge come from? All over. And all of it can have its own set of rewards.
Talk with fellow collectors - especially when you're starting out, visiting other more experienced collectors can be very helpful. First, there's the sense of camaraderie that comes from "talking shop" with someone who's just as into a subject as you are. You can also see what your collection can potentially grow into, look at display and organizing solutions, and so on. You might see fairly rare pieces up close. And by talking with other collectors you'll generally get some hands-on tips about what to look for, what things are truly worth and so on.
Personal experience - over time, you'll become one of the collectors I just talked about in the paragraph above. Chance are you'll get burned on some purchases, you'll learn the hard way that all that glitters is not gold (or even pyrite). If you learn from your mistakes, you'll eventually develop an instinct that will help you when you encounter something out of the ordinary.
Case in point: at a recent toy train show, someone brought in an unusual piece: a vintage Lionel steam engine with an "Eastern Railroads" decal on its tender. It was decided a one-of-a-kind item, so looking it up online or in a reference book wasn't going to help. First off, what was it, and secondly, was it real?
I happened to be there during the discussion when this item was presented, and the collective wisdom of the folks examining it went as follows:
"Eastern Railroads" a road name used in the "Railroads at Work" diorama at the 1939 World's Fair Railroad Building (sponsored by the Eastern Railroads Presidents' Conference).
Could this be a surviving piece from that legendary display?
No, because the engine was an off-the-shelf O-gauge locomotive. And while it was made in the late 1930's the railroad exhibit used all hand-made smaller scale models.
Could it have been a display piece from some other part of the railroads pavilion?
Possibly, but the trim actually dated the locomotive as coming from a run made after 1939, and therefore not likely to have been at the Fair.
What about the paper decal?
Decidedly of the right age, but hand-applied. And such decals were known to exist.
Most likely explaination: someone with connections to the Fair had obtained the decals and made their own souvenier by converting a locomotive they already owned. There was probably no intent do defraud, and so this is an interesting curiousity, even though not an actual piece of the 1939 American Railroads exhibit.
All of which, of course, greatly impacted the value.
Reference Works - reading up on the subject of your collection can help greatly in several areas. Sometimes the way a reference book is laid out can give you ideas on how to organize your collection. And while it can give you relative values, don't take them to heart -- most price guides are out of date before they roll off the printing presses. Such guides can, however, be useful in helping you understand what's out there in your field, and how available (or scarce) particular objects might be.
Historical reference works are useful, too. Lists with prices are great as shopping guides, but to really understand the nature of the objects in your collection and the reason why some are more valuable than others, you'll need to understand the background of your objects.
If you're familiar with the history of Arm & Hammer, you'll have a better idea of why tins from certain years are more valuable (and more desirable) than others. And if you're collecting any type of object from the past, just understanding the general history of the time will help tremendously, too. The scrap tin drives of the Second World War took a heavy toll on inexpensive prewar toys.
And don't forget original sources. Again, if you're collecting something from the past, company records may be spotty or even non-existent. The best references for 1920's Tootsietoy dollhouse furniture are the catalogs the company sent out to retail buyers. You'll see what was available, when it was available, and in what colors. If you look at successive years, you'll know when items were discontinued, redesigned, recolored, and/or repackaged. All of which helps accurately date objects when you find them in the wild -- the very kind of information used to determine that the World's Fair locomotive couldn't be authentic.
If you're collecting your passion, then all of this research -- both written and oral -- will just add to your appreciation of your hobby.