Something i have found extremely useful is little pully system even Bruce Lee used... I cannot go all the way down into the sidesplits, im about 5 inches from the floor in the front splits... but about about a foot and a half in the sidesplits.
Anyways, something ive found helpful is making some sort of pully system and use some ribbon or anything that will hold your leg in the air comfortably. then what you do is you put your foot in the noose you've made and then pull the otherend of the ribbon so it raises your leg high into the air, its helped me alot with the sidekick aswell as doing the side splits.
You may find it silly but i find it usful.