You can get special onion fertilizer which is high in all nutrients but seems to be especially high in potassium. NPK analysis usually around 12 12 18. Sulphur is important too.
I've switched to heat treated sets. Had a good crop last year. We are still eating them from stores although they are growing out now. My other thought is not to set them too early. Last season I worried about being a bit behind but in the event they seemed to miss some of the early season pests.
The thing is, despite all our best efforts, a big element is down to chance. The weather, rainfall, sunshine and pests can all combine to give a brilliant crop or a disaster. Fingers crossed.