Do you mean how to keep the
crows attracted? Right?
Anyhow, making friends with crows can sometimes be hard, depending on their experience with humans. Crows that have been harassed by people or shot at (etc) are far less friendly and they actually teach that to each generation. There have been studies shown that crows recognize individual human faces and can remember them for years and will teach new generations about it as well! So it's important not to upset them if possible.
Like all animals, crows are very food driven. They adore meat scraps. I have a wood pile behind my house that I always put the meat scraps (that the chickens don't get!) on for the crows. I will also leave them stale bread, though not too often as it's not overly good for them (people might think I'm nuts for caring, but that's just who I am). It took them a week or so but they have learned that it's a buffet, and that I am never threatening to them. I have lived here for three years and it's taken that long for the crows to trust me enough to stay in the yard when I am out and about. I live in the country, though, where people have shot at them before. In more urban and suburban areas, crows are usually less fearful of people. They've started nesting in my pine trees, which is great.
A few hints about leaving meat or bread scraps out: Always leave them in the same place, visible from above, so that the crows learn to look there. A stump or a log or whatever works well.Crows learn our human routines pretty quickly.
I won't feed my chickens raw poultry of any type for fear of disease vector, so the raw poultry trimmings always go to the crows.
Other critters love meat/bread too-- you don't want to attract more
predators to your yard! For this reason, I always put my "crow food" out in the morning, so that they have time to clean it up before nightfall. Once they find the snacks, it rarely takes them more than 30 minutes to clean it up. I've never had other critters come in for it, in this way.
If you can get crows nesting nearby, that's the best! Crows are very defensive of their nest sites and will brazenly chase hawks away during the spring and summer months. If they know they have a good food source and there are any pine trees nearby the chances are higher.
Best wishes!!