There is a problem with that lip all together. Because it's so low to the ground, the material needed to be very bendable so it didn't break if someone ran it into the curb.
There is 2 things I'd do.
1) Get under the valance and make sure it's wedged into the bumper. There are 2 locations where there is like a little flat blade sticking out that you can slide the valance into. Make sure it's tucked into there and then clip the valance up.
2) Grab yourself some of the 3M double sided sticky tape and slap it between the bumper and valance. You shouldn't have any issues with it afterwards. I'd sand the valance down where you're going to apply the tape tho, as the material isn't the easiest to get anything to stick to.
Unfortunatly this is going to be a prob with most of the valances because of the material used. It doesn't have the memory of most plastics therefore is going to sag.