Go to Promotions -> Shopping cart price rules and add new rule
Set the following:
If ALL if these conditions are TRUE
Select from drop-down - 'Subtotal'
Change 'is' to 'equals or greater than'
instead of dots put 500 or 1000
Apply: Percent of products price discount
Discount amount: 5 (or 10 for second)
Here is the article where this case is explained and there is also 6 more price rules that are most often used -