My fiance and I use this app that syncs between our iPhones. Whenever I add something, he can see it on the list and vice versa. So whenever we realize we're out of something during the week, we add it to the list and then refer back to it when we go shopping on the weekends. It works quite nicely most of the time but my fiance doesn't always check it if he does midweek shopping to see that I need milk. >.<

When we shop on the weekends, I try to get everything on the list but sometimes when things are really expensive I hold off buying it till the next week in hopes that it'll be on sale then. I also buy stuff that wasn't necessarily on the list but is on sale or I realize I need/want it. We've had to be more careful with our spending these past few months though since my fiance is currently out of a job.