Hmmm I don't tend to wait for sales as I have a lack of patience and also fear that my size will sell out. If there's something I'm being indecisive about and then it goes on sale, I'll be likely to go and try it.
I don't really plan much in advance, I manage to save most months and have disposable income (still living at home!) so normally I'm ok.
Every purchase is planned though, so in most cases I'll find something I like online, and think about it for a while. If it's a pair of Diesel jeans I'll look for other pictures in different lights, fit pictures etc. Once I receive an item I try it on to make sure I'm 100% happy with it. If not, it gets sent back, so I'm never (or not usually) spending money unnecessarily. Have been a few items which I've liked and then gone off a few months later though, I can't deny that!
At the moment though I've got a few pieces that I want all at the same time, and they're all quite expensive. Flyknits, Diesel hoodie, Diesel jeans. I'm sort of just delaying purchasing them, don't really know why just can't bring myself to splash all that cash just yet. I think I'll purchase one of them this month and then the other one or two after I get paid.