Have you tested to see whether your trolley jack will actually fit under your car,as a lot won't.
If it does,be careful that you don't crush the (4) plastic jacking points with the jacking cup.
To get around this,as mentioned by OB,the cheapest option would be to cut some pieces of wood to fit inside the jacking points or to buy a custom made adaptor(eBay,approx £18).
I've used both as a garage crushed my jack points.
If you get on all fours,this is what you're looking for-just behind the side skirts-hth