I too have rules, both for building the kits and to control my stash.
I have two major obsessions, so I usually build models in batches of three: an F-4, a Spitfire and one other. I never allow myself to have more than three models on the go at the same time. I also never allow myself to have two or more of the same type on the go at the same time - I learned that lesson when I got dreadfully bogged down building five Bf-109s at once. The rule I use when choosing my next projects is that I am not allowed to build a model in a similar colour scheme to what was built in the previous batch. So, if I've built an F-4 in the classic USN scheme, I cannot build anything else in a similar scheme in the next batch. This just means that I don't get bored with seeing the same colour schemes on my bench all the time.
In an attempt to limit stash growth beyond that which is sustainable, I've come up with a number of ridiculously complex rules to ascertain whether I can buy a particular kit or not:
F-4 In the stash, I currently have 34 F-4 kits, 23 of which are Hasegawa ones of various types. The remainder are Fujimi Brit Phantoms. I'm not allowed to buy any more F-4s unless they are the Hasegawa F-4G or F-4S re-releases. In which case I'm allowed to buy no more than two of each.
Spitfire If the number in my stash slips to 2, I can buy a number of others, but no more than 3, giving me a total of 5. I can only buy those three after I have built at least two other kits.
Others If the kit level of other obsessive types (Hunter, F-86, F-16, A-7) slips to 2, I can buy no more than 2 more, but only after I have built at least two other kits. In any of the stated cases, that's unlikely to happen for some time.
If the kit level of other non-obsessive (as yet) types slips to 1, I can buy another, but only after I have built at least two other kits. This is an attempt to ensure that the non-obsessive types remain non-obsessive.
If a kit in my stash shows signs of being taken off the market or selling out (I'm thinking of the MPM/Xtrakits Meteor here) then I can buy up to five of them. F-4s are not included in this rule.
Whimsical purchases I can buy anything that takes my fancy, provided that I build it immediately. This allows me to buy such gems as the Eduard FW190, but prevents me from buying more than one at a time. A whimsical purchase is specifically excluded from the above rules. For example, after building that FW190, I can't go straight out and restock it, placing the new one into the stash.
OK... there are my rules. I intend to stick by them. Surprisingly enough, they have worked for a few months and although my stash hasn't reduced dramatically, it also hasn't grown above my self imposed limit.
And be aware that this only applies to my model aircraft stash. I also have a model railway stash, and a Deep Stash comprised of kits that may never, ever get built...
1) Never buy Testors, Italieri, or any other kit where I can get the same subject from Hasegawa or Tamiya or Trumpeter.
2) WWII in 1/48, jets in 1/32...though I've been breaking this one of late because so many great 1/32 WWII subjects are being released.
3) Never build without using photo etch.
4) Never use kit decals.
5) Never build without research.
6) Never build when distracted or I don't feel like it.
I wish but presently the majority of my collection is unfinished projects.
I tried sticking to the Rule of Three but then I started multiplying threes.
I do try new things but not that regularly.
I started messing with vacuforms back in the '70s when Rareplanes hit the market. I finally got one finished about 5 years ago and that was more of a conversion than a straight kitbuild. Currently I have an unfinished McDonnell FH-1 from Formaplane about halfway.
Do you feel pressed? If you're enjoying it, keep at it until you don't.
As for my rules - If the kit is not in 1:32 don't buy it (Unless it's for my son). If you have two or more identical kits, they need to be converted og modified into something else, three Hawker Typhoons,one OOB, one with Paragon Bubble top and one backdated to Tornado, Three P-47 (Revell) one converted to N version with long wings etc..... Also after one of those series I often do one OOB,or even better, do a "What If" like X-1 fighter, Me 262 from Aeronavale Suez period.