Well, there are a couple of things which you can do in order to ensure that you will not waste any food you buy. I live alone myself at the moment, but during the last couple of years I had always had some flatmates. Anyway, I can understand well what the problem is.
Firstly, if you cannot do it on the fly, you should plan every week, let's say on Sunday since it's free and you can easily spend some time. Also, I don't do shopping on Sundays usually and it's like a day of break during the whole week. There's nothing against it then.
Some tips for planning:
1. Think how much you eat and therefore, how much you might need.
2. Prioritize what you eat, start with the stuff which due date is the shortest one coming.
3. If you cannot eat something, share with your flatmates or invite your friends.
4. If you can't do what's been written in number 3, you can freeze (as long as none of the ingrediends
5. Always segregate your organic waste so it wouldn't go for nothing when you throw it away.
I'm sure you're gonna do great!