I hereby admit that I am incapabale of patching a tube so it wont leak. I think I am doing it right -- scuff surface, apply vulcanizing fluid, let dry, apply patch.
I've found that the 'let dry' step can be important and therefore it's best to do the patching at home without any time concerns. So I carry a spare tube to change the flat on the road (and also a patch kit just in case there are multiple flats). Once I have a few tubes to fix I do the patching at home and let the glue sit for quite awhile to dry before applying the patch. This also allows the patch job to cure fully before it's put into use. And I agree with a previous post that it's best to remove the cellophane covering from the patch.