So instead of getting a new foundation I use the foundation that I already have and mix it with a liquid highlighter, I use Benefit's High Beam or Moon Beam, there are loads of cheaper alternatives on the high street, I think mememe has a copy of High Beam. Although you are buying another item this will be dual purpose instead of buying a foundation you might not use much.
I put the normal amount of foundation on the back of my hand and mix it with a smaller amount of highlighter as you don't need much because it goes a long way.
I love doing this on a night out, or if I think I look tired, and people have commented about how good my skin looks. You could also do this with BB creams or moisturisers if you don't like a heavy feel on you skin.