Believe me it's not hard. I've spent 15 years trying to get rid of them.

Just drop a berry on a bit of ground where normally nothing would stand a chance. Turn your back for 5 minutes then when you give a startled glance behind you because something has smacked you hard between the shoulder blades and blocked out the Sun whilst lacerated your leg don't be suprised.
It's Godzilla the Blackberry thicket.
The plus side is you'll have no problems ensuring an active old age. You'll be fighting it back until you die.
You'll also be sick of Blackberry Jam.
If anyone says 'Summer Pudding' in your hearing you will vomit into the nearest receptacle.
You-have-been-warned.