what helps me slide longer is trying to look at the end of the flat bar, like over your shoulders (if you wanna come out regular at least), your arms should be over/paralel to the rail, and perpendicular to the board. you have to commit your bodyweight or else it will slip out. I imagine myself trying to move just my legs when im ollieing into the flat bar, not like a backside 180 where you turn your shoulders. try to go little by little adding some distance. speed helps but also makes it scarier... try to find your happy medium.