How can give buy 1 get 10% for selected products in selected categories.
You can make good use of Shopping Cart Price Rules. You just need to set the criteria on Conditions tab. Add a rule, specify Products Subselection, then choose 1 on If total quantity is. Then add conditions under total quantity and select SKU under Product attribute. Finally, switch to Actions, select Percent of product price discount and enter 10 on Discount Amount.