It's hard to anticipate whether an item will go on sale or sell out. I usually don't try to time sales--if I love something and think I'm going to have it for a long time (ie, it's not a "trendy" item), then I just end up buying it when I have the money saved up, rather than waiting for it go on sale. Sometimes the item ends up going on sale after I buy it, and sometimes it sells out. But because I really love it, it doesn't really matter to me either way, I'm just happy to have whatever the item is. Does that make sense? Are you a student or teacher? If so, you can get a 15% discount on full price merchandise at J. Crew.
Also, if you are going to wait for it to go on sale, keep an eye on it--if you see other sizes in the same color selling out, then you should probably jump on it. If you are a smaller size, 0, 2, 4 then I wouldn't wait, those go fast.