There are two classic ways to solve this. First either use heavy throw pillows so they don't slide as easily. Or second, just stack pillows atop one another for weight.
Well you can't really keep them in place they usually slip off and it depends on the material of your slip throw. You can just opt to buy some cotton ones.
In order to keep sofa throws in their place on the couch, you will either have to start buying heavier throws so that they don't move around too much, or else weigh your current throws down by stacking them.
