Mary isn't really given that much credit in Hoodoo. Not many people work with her, and I think it's because they have a hard time identifying with her associations - or think outside the box about them.
Mary is the Saint for penitent sinners and converts.
Now, converts we can understand easily what that means, but some folks trip up on "penitent sinners". According to Catholic traditions, we are all born sinners whether or not we recognize it. So, being a penitent sinner simply means being sorrowful for your sins or wrong doings.
So Mary can be petitioned when you are feeling guilty about actions you have taken. She is great for self healing and forgiving yourself. Because she was so close to Jesus and the first to see him resurrect and give his Easter message, it's believed she has his ear.
Due to the theory of her being the wife of Jesus, she is also sometimes petitioned for luck with love and marriages.
As for offerings, spikenard, perfumes and red. She's been depicted with red eggs (speculation is that the red egg symbolizes her fertility, and consequently offspring, with Jesus).
Keep in mind that by working with her, she may be willing to help with other matters as sometimes Saints do with people who are devoted to them or are strongly called to work with them.