In this Instructable, talbotron22 shows how to make "Kitty Crack," an ultra-potent catnip extract containing nepetalactone, catnip's active ingredient. One pound of catnip yielded 143mg of nepetalactone.
A note about safety. Yes, it is safe to use this extract on cats. I have looked into it, and there are a number of studies (very interesting in their own right) using pure nepetalactone on cats in experiments trying to figure out why it causes them to go bonkers. The upshot is that it's pretty safe. In the last of the references below, the LD50 of nepetalactone was determined to be 1550 mg/kg (about the same as aspirin), meaning you would have to force feed your average 5 kg cat ~8 grams in order to cause it any harm. So as long as you are reasonable with the extract it should pose no harm.