I think with any hobby involving the collecting of something, the main rule to stick with is to only collect what you ACTUALLY like. This may sound redundant but it's so easy for collecting to turn into hoarding. And I don't mean having a house filled with piles a shit and and only an 8" walk path. I mean limit your collecting to what you truly like and not just what you can get.
I've been a toy collector since probably the late 70's. Even as a 4 or 5 year old I remember feeling I had a toy "collection" and not just a room full of toys. I played with them but I didn't beat the shit out of them either. The bo-bo drugstore $1 action figures were the ones that took the firecrackers and BB gun abuse. As I got older and started working, I had the funds to really build my collection especially with local antique toy shows popping up. And even tho I'd branch out a little with Japanese robot wind-ups or various other toylines, I'd always stick with the backbone of my collection, 3.75" action figure lines (GI Joe, Star Wars, Battlestar, Buck Rogers etc.)
Around 1995, Star Wars toys made a come back to stores and a few years later Star Wars shit was available everywhere. Underwear, paper plates, curtains, 12" dolls, Jello molds etc. I met and heard of so many new collectors that went fucking apeshit buying everything they saw with R2-D2 or Yoda on it and within a few months they were burnt out. I only bought or hunted for what I liked. If Hasbro made figures that looked like ass, I didn't buy them regardless of how rare they supposedly were. Other folks would do toy runs at 3 am to every Walmart in the area and come back with 2 monkey faced Leias and they felt like they found the Holy fucking grail. They didn't really like what they bought, they would just buy for the sake of having.
This same collecting frenzy happens with any type franchise, Nascar, Beanie Babies, Kiss, Barbie, HotWheels, Twilight, Harry Potter, The Beatles etc. So many folks will just buy it because that roll of toilet paper is printed with Dale Earnhardt's face on it and they'll put it on a shelf right beside their 47 boxes of misprinted Mr. T cereal.
As with arcade collecting, I say stick with the same rule. Only buy the games YOU really, really want. Not what's rare or what everyone else wants but what makes you honestly happy to own and not mind staring at everyday. If games can be bought cheap and flipped for a profit to go toward your dream game then great but it has to be carried thru. Hoarding up games in shit condition with no future of ever looking decent or being playable again will surely throw a wet blanket on the fun and thrill of collecting.