Mix and match
Different species prefer different foods. Goldfinches, for example, are very fond of niger seed. So, match the food you put out to the species you know already visit, but also think about trying different foods now and then to attract new species. Basically, the more different types of food you put out, the more species you’re likely to see.
Splash out
Don’t forget that birds need water, too. Keep a bird bath topped up with fresh water, or even just an upturned bin lid, Frisbee, or similar. Of course, this is all the more important in periods of hot weather and low rainfall, when puddles etc., aren’t available.
Location, location, location
Put your feeders where the birds can’t easily be stalked by predators, but close enough to…