If you get a container of dried catnip, this is the form we use, you can either dunk plush toy into it, or sprinkle some onto an old washcloth. We've also stuffed a sock with catnip, tied a knot in it. My kitty likes to sleep on his sock.
I know many toys say they are infused with catnip, and some my cat responds to, but I think the catnip does loose it's "flavor" over time. When you have the dried stuff, you can quickly remedy that, however.
I've once tried a synthetic catnip spray, and perhaps it was because my cats were used to the real stuff, but they didn't respond to the spray on stuff at all.