Heater-A/C ducting tips & tricks...Please

[DT1]

I have a new set of heater and A/C ducting and I'd like to know if there is a way to expand them to fit properly. I have never been able to put them on without slitting the ends and duct taping them in place. Is there a trick to it? Do they need to be per-expanded to go on? The only thing I can think of is to push them over a conical shaped object to slightly increase the diameter so they will go together easily. Maybe wetting the ends will help. Any advise will help.
Thanks in advance!