these took me YEARS to get. YEARS. i had a bunch of frontside tricks down, but backside just didnt work for me for whatever reason.
just takes lots and lots of practice. think about ollieing higher than you need to, i think thats what got me grinding on this one. i think its hard because frontside you can see everything happening in front of you, backside is more about having the feel for it and being confident. like people have mentioned, ollieing up curbs backside will definitely help you get the fell for backside tricks. once you can do that really easily, its not too hard to shift your shoulders and get your trucks on there.