I usually have a wish list and depending on my mood, I buy what I like the most that day. Whatever clicks me. I've filled my cart many times in the morning and then after a few hours, I check it and I end up buying different kits. I start opening tabs with the ones I want to buy and start checking out the layouts, to see how many cute elements or papers each kit has. Then if I can't buy them all, I start counting which ones have more of what I like. It's always a bummer when I can't buy everything I want and I have to chose between them, because they're all amazing!! This technique always helps me narrow them down to the amount I have available to spend.
Another trick I do is I don't always buy the collections, if I want more stuff and want to have a variety, I buy only the kits. That way I save on money and get more variety. I wish I could always buy the collections, but I'm satisfied with the kits only too.