The best way is to just do it. Lots of it. Particularly in a situation where you just have to keep going, like playing duets, playing along with a record, etc. Good sight reading involves several different skills. One is the ability to look ahead of what you are playing so you have an idea of what’s coming up. The second is the ability to see patterns so that you read not just individual notes but also harmonic progressions, scale patterns, etc. The third is to be able to remember, just for a second, what you just looked at so you can play it while you are scanning ahead. The best sight readers are the ones who do it all the time, like symphony harpists and also pop harpists.
I’ve found it really useful to go back and play something I haven’t played for a year – so I’m pretty much sight reading, but the fingers start to remember and I start to recognise the tune (thus making it slightly less frustrating that trying to sight read a random unknown tune!)
Learning the patterns on the harp is such a valuable tool. Once you recognize arpeggios, 4-2-1, chords, etc. the left hand or harmony (sometimes this is in the right hand too)becomes very easy to sight-read. Then it’s just a matter of looking at the melody. I’ve been really working on this myself, re-training myself to sight-read more rather than memorizing. I can manage simple pieces pretty well, intermediate is still a challenge though.