How to get IFR Clearance enroute?

What are the recommended procedures in obtaining an IFR clearance
enroute?

Flying from NJ to FL, I prefer to fly VFR, but undoubtedly I will
encounter IMC along the way. If I filed IFR in advance, it takes 1/2
hour or more to clear it through ATC. I obviously cannot wait 1/2
hour if I encounter IMC enroute.

Is there an "acceptable" procedure enroute if I start seeing a cumulus
build-up in front of me? Do I have to land and file? Do I file with
FSS and wait 1/2 hour before I open with Center? Can I file directly
with Center? Is this idea generally "frowned upon" by ATC?

I'm not necessarily looking for regulations as I am for practical
experiences that others have witnessed.

Thanks for any tips.

Stimbo
Medford, NJ