On YC, it sometimes helps to simply sift through a user's past posts and comments.
If you concentrate on the top of the new page and like a submission and mod it, it will get exposure near the top of the top page and get some real good input usually, so that's the best place to start- just look at the 3 or 4 submissions at the top of the page and spend a few minutes.
Nevertheless, there are some amazing submissions that because they don't get a point in their first hour of existence (because everyone is eating lunch or busy submitting their own etc.) end up moving clear down the list without anything. If you want some good karma (the real kind) spend time with the 4 or 5 at the bottom of the list giving belated deserved credit for some posters.