I think that all of us who have been around 10 years or more have had a tendency to evolve and change with an ever changing market. As inventory had dried up in other series, and exploded in some others, I have always moved back and forth between a few core series and then slowly added more as inventory has come available.
I would say my core when I started was Sailor Moon and DBZ, however when I started DBZ was in one of it's pricier times so very little was added to my gallery at the time, prices are similar now to what they had been then. Sailor Moon was pretty competitive between 2002-2007, I bought a good chunk of my collection then but the inventory even then was drying up slowly and prices were high to middling, nowhere near the rumored heyday of the late 90's.
In 2004, I added GW and CCS to my repertoire, with a brief dip into Tenchi Muyo! which lasted like a year, CCS was UBER cheap back in the day... well except for SCENE cels, although those were slightly cheaper than they are now. In 2006, I sold off most of my GW collection, except for 3 pieces or so.
In 2007-2008 I bought my first Saiyuki Burial Artwork off of Mandarake there were 3 or 4 BG/Layout Lot auctions from the very first episode of Burial I had won 2 of them. I still regret to this day not just plunking down the money and winning those other lots when I could have; as the collector that won them has gone down into the black hole. This was the start of the Burial Obsession...
From 2008-2012 I was generally lurking not really buying things (maybe a few here and there), and then in 2012 I came back...
This brought an explosion of purchases on my part. The market for Sailor Moon and DBZ was in a lull... I was able to add a ton of items to my gallery for prices I did not think possible in years before. I also started stalking Saiyuki in a way that I hadn't before, it was added to my list of series I would regularly check. I also in this time frame started collecting Tennimon, and let me tell you if you start collecting Tennimon plan on donating some of the extras you receive to cons and the like they propagate like rabbits.

*prolly has 100 Tennimon cels/sketch sets, and could prolly donate half of them... still.*
After the flurry of Sailor Moon, Saiyuki, DBZ, and Tennimon stopped, Sailor Moon basically started to dry up, most things I have recently purchased from Sailor Moon have come from Private Gallery offers and Mandy, YJ has not had anything recently that has tempted me, except for a few pieces which are few and far between I think I've added 3 or 4 last year. DBZ has shot up so much that it is better I cut my collection back some more and wait for the market to back off from it's current craziness, so I am cutting back there. Tennimon is sporadic as well as what I have is really nice and the cel has to be really special for me to add it to the rest of the bunnies.
In 2014 - Present - I have from the Saiyuki obsession discovered a love for sketch art, so I branched out then into Loveless, Junjo, FMA, Bleach, and Sekai Ichi Hatsukoi. I'm sure there are more lol, but I have been more aggressively looking for newer series, as well as spending more time meeting up with other collectors at cons in the like, also original art is a new obsession... *blames Cutiebunny for this, but also snuggles her for this* This is in part a good spin off of my love of Production Sketch Art, and seems a natural progression from there I think. In the meantime, I mostly still stalk, Sailor Moon, CCS, Tennimon, Saiyuki, and Loveless. Most of the other series that I collect from I am checking more sporadically, and I also check AB and RS regularly to see what offer periods come and go. A lot of my recent purchases have been made this way, and I am close on some payment plans that I started late last year that I will be finishing soon.
Sorry for the rambly dissertation... I get that way when I reminisce... *is old lady*
"Any idea is a good idea except the non-happy ones. Those we push down deep inside where you'll never, ever, ever, EVER find them!" - Princess Unikitty
http://www.jadeduo.net Please Stay By My Side, In My Dreams - Sailor Moon, DBZ, Cardcaptor Sakura, Saiyuki, Gundam Wing, Sekai Ichi Hatsukoi, Junjou Romantica, Fullmetal Alchemist, Bleach, and Tsubasa...
Actively Searching for: Saiyuki Burial (Anything), CCS Episodes 65-67, and Primo Sailor Jupiter Cels
Anime-Beta Annual Cel Contest - Grand Poobah, 2014, 2015, and 2016
Anime-Beta Annual Sketch Contest - Grand Poobah, 2015
Anime-Beta Annual Sketch Contest - Survey Wench, 2016, 2017
Still lurking around Beta, but most days you can find me in the Cel Chat...
http://cels-chat.herokuapp.com/ Come Join US!
Also rebooted the FB page, you can find it here:
https://www.facebook.com/groups/348416495554600/