How can I stop a barbed coupler from leaking?
That looks like a very flimsy coupling, as it barely has anything to hang on to. Better to use a regular PVC barbed coupling, which looks like this:



Usually you use a stainless steel clamp around the connections:



With bigger/thicker pipe, you sometimes need to heat the pipe using a torch slightly -- just get it a bit pliable so you can get the fitting in, but not "melty". 

Here's a finished connection (using "white stripe" 75psi poly - in this case as a non-pressurized drain line):



Here's another example showing connections to soaker hoses: