Add details with a filter
Another technique I like to use is the Poster Edges filter (Filter>Filter gallery>Poster Edges). When I’m close to finishing a painting I will duplicate it, flatten all my layers and run the Poster Edges filter. It crunches all the edges and can be harsh at first, so make sure to reduce the Transparency by 60-80 per cent. The result can sharpen areas and give you more detail without a lot of work. You may have to erase areas where the filter isn’t working, though.