Yes, to give you good advice we need to know what failed and by how much it failed compared to the passing standard. This is usually detailed for you on the test results.
Depending on what failed, I usually tell my friends some of the following things:
1) Start running premium fuel for a few tanks.
2) Run some injector cleaner through the system at double strength. Or carb cleaner if the car is older.
3) Just before the test, change the oil and use a heavier weight oil if now using 10w30. Try 15W40 or 20W50 in summer time. During the oil change, add STP Oil Treatment to further thicken the oil.
4) As mentioned, if you haven't changed the plugs in awhile, do that.
The point is, you may not have a true catalytic convertor problem. The car may not be burning fuel properly, or it may be burning just enough oil to mess up the test. My suggestions are aimed at burning fuel properly and reducing oil consumption.