Interesting question!
I belong to a club (now a museum, actually) with a 'UGE layout so I tend to go for quantity rather than quality. But boy, does the quantity add up fast!
My budget does not allow for much buying new, or maybe I don't allow it to allow for that. I can't see paying $300 for a locomotive. So a lot of what I buy is new-old-stock (i.e. Proto blue box), or items that are on sale at TrainWorld or Walthers.
That makes collecting a bit more of a challenge, but also good fun. I wanted a Phase I Amtrak train, but didn't want to spend $80 per car, so I took my time, kept an eye on the sales, went to train shows, checked in on hobby shops as I traveled. Took 2 or 3 years but I ended up with the train I wanted, including a pair of Proto Es to pull it. (I'm still adding cars as I find them.)
And sometimes I get -really- lucky. A few years back Walthers put its Amtrak Proto Hi-Level cars on sale for $22-$23 each. I got a set of 6 cars for less than the retail cost of two! I can run it as a six-car train, or add my Amtrak Heritage fleet cars for a Southwest Limited. A friend at the club found me a pair of Athearn SDP40Fs on the cheap. This train-building fever is contagious!
I can't always find what I want, but I can often learn to want what I find. I stumbled on a great deal on old Proto 1000 Erie-Builts. Read up on Fairbanks-Morse and fell in love. Wound up with an A-B-A set of Eries, plus an A-B-A set of C-Liners with an extra pair of shells in a different paint scheme.
How do I keep it new? One way is I rotate. The freight cars stay on the club layout, but I rotate my locomotives and especially my passenger trains. I'm about to pull my heavyweight NYC off the club layout and put on that aforementioned Amtrak Phase I train. It hasn't run in about two years so it'll be new to me!
I also look for different combinations of the same equipment. I mentioned running my Amtrak cars either together with Es or with the Hi-Levels and SDPs. I'm working on a NYC Budd train (Mainline cars modified with grab irons and people). It's not done, but I can combine a couple of its coaches with a PC green sleeper and bag and a just-acquired NYC grille car (Thank you Ed GMPullman!) and my PC Bachmann GG1 (on sale at TrainWorld), and hey presto, it's a just-post-merger Penn Central train!
Likewise, a friend just passed away and left me his mostly-correct 1948 20th Century Limited. We've been running it on the club layout for ages. By pulling the lounge, adding one of my Great Silver Fleet coaches, a Slumbercoach (thanks again, Ed!), and maybe a silver 10-6, and swapping one of the E7s for an E8, now I've got a 1960 20th Century limited. Or the Century's sleepers can mingle with my Budds, and now I've got a nice generic NYC train from the 50s-60s. A diner and heavyweight baggage cars puts it in the 1950s; fewer sleepers, a grille instead of a full diner, more head end cars (including FlexiVans) and a cigar band shell on an E-unit chassis puts it in the 1960s. All new trains to me!
Aaron