I tend to use a deeper dish rather than just the lid. That way you can get maybe 2-3 inches of water in it as apposed to about 1/4 of an inch in the lid. With this set up if you stir the oil in a bit you'll get some suspended in the water and not just sitting on the top (you have to be quick mind before it all floats up again
). This'll give a more 3D effect adding depth to the pic.
You also need to raise the dish about 6-9 inches above the card (I use DVD covers for this).
I've looked at the pics in the link and think there are still focus issues. Dunno if you're on af but as with most Macro stuff I'd suggest mf. With the above set up it's easier to focus on the oil as it's a fair bit detached from the bg which also helps with dof if you are using a large aperture.