My MAC rules
1. I try very hard not to buy anything I know I won't really use. For example, I do not collect any of the following:
* TLCs in pots (I do want the sticks, though)
* Lip palettes
* Brush sets (I have more than enough brushes, MAC or otherwise)
* MSFs (I adore them but because they last forever I only have Shimpagne and might get one or two more in contrasting colors, but am going to avoid the "collect them all!" hype)
* Full-size pigments. I have TONS of samples and doubt I'll ever run out. I let myself buy samples as I please because they're cheap.
* Paints. I have Stilife; that's enough for me. I am starting to have a shadestick problem, tho
* Permanent eyeshadows. I have two palettes' worth (perm and LE/DC), and with a few small exceptions will only buy LE ones from now on *if* I don't have something really similar already.
* Lipglass. They don't last too long, I have more than I need, and I'm not in love with the texture. I do like lustreglass & lipgelee, but since I have the holiday gelee set I don't feel I need many more. (Wish I hadn't missed WTL, though.) I let myself buy lipstick pretty freely, esp. w/B2M
If I like a lippie, I buy it. If I can get stuff used, so much the better.
* Short-lived eye stuff, like liquid/glitter liner (I only have 2), mascara etc. I can see myself collecting f/l but am trying to bereasonable since you can always layer shadow over blacktrack or pencils. I do love the kohls & powerpoints and since they practically last forever I let myself buy what I like.
* I don't collect lots of body-shimmer products - pigments & MSFs are enough for me, plus I'm 28 years old and work in an office and can hardly go around sparkling all the time.
* Glitz glosses. (I have one, but that's my sole exception. I'm not fond of lip stuff in pots.)
* CCBs. They're pretty, but I hardly need mroe bases. Stupid me just bought one off eBay in an impulsive-buy moment; it will probably get resold.
* "Gimmicky" stuff, like those petit-glosses with the icing. Cute, but a. it's a lippie in a pot, so no good for me, and b. way too cutesy for my taste.
* I collect the postcards, but not obsessively - I won't pay crazy prices on eBay for the old/non-U.S. ones no matter how cool they are.
2. I try not to buy LE/DC just because it's LE/DC. I've made exceptions, but in looking at the upcoming collections I think I'll be very selective. (The exception this season wil be chromeglasses - OMG they look like pure love!) Icon, for example, is gorgeous but likely to be too warm for my coloring (NC15). Sweetie Cake is a bit too cutesy for me, but the pencils look like fun. Culturebloom, however, may present a serious temptation. :-/
3. I try to evaluate what I have and don't need on a regular basis, and sell off what I don't/won't use.
4. I try not to buy something that looks like what I already have or could be recreated by layering colors. Of course I've broken this rule a bunch of times, but it helps me sort thru new collections.
5. I DON'T BUY BACKUPS, EVER. NO EXCEPTIONS. Something will always come out later to replace anything I use up, and the only product I regularly use up is pressed powder! I have enough makeup that running out of something is highly unlikely.
Uh, these rules subject to change...