As a string player, I found playing in small and large ensembles very helpful for getting more solid with sightreading. Playing with others forces you to keep going even if you missed a little something along the way. And it helps you get in the habit of looking slightly ahead so you aren't too surprised by a sudden key change or whatever.
Pianists are at a disadvantage in this respect, working alone most of the time as they do.
Frankly, if you are at least at an intermediate level, finding some ensembles to play in, or even just a friend to play some duets with, would be the most helpful thing you could do.
If you are somewhere in the beginning realm, I would recommend a series of sightreading training books by Paul Harris.
It is also helpful to keep the key you are in clearly in mind. Identify the key, play the scale, get the tonality solidly in your ear, and make sure you know which black notes are part of the key, before you start playing.