When you boardslide, are you falling backwards? You are probably straightening your legs and leaning back, away from the direction you are headed.
When you learn how to snowboard, you learn to always tip up your board when your belly is facing downhill. Keep the downhill edge up so you don't catch it. This works against you when you are learning to boardslide on boxes and rails. Instead of lifting that downhill edge, you have to keep your board totally flat. Think of the rail as like sheer ice on a hockey rink that has just been zambonied and if you tip your board at all, it will slide out on that ice.
A trick that a friend told me when I was just starting to learn boardslides (I know this sounds crazy, but it worked so well for me!) is to pretend you are bending down to pet a golden retreiver (this particular dog works because of it's size). This shift in your weight sets you up perfect to keep that board flat so you don't lean back and fall on your butt on the rail.
Also, keep those knees bent! Exaggerate it at first if you need to. A lot of times people fall because they straighten out their legs, which in turn causes them to lean back.
As far as if you have to jump up, spin 90 degrees and land on the rail in a boardslide or not, that is totally up to you. That is one way to do it, but an easier way to start is to do it when you are already on the rail. Get on the rail in a 50-50 then, keeping your shoulders lined up with the rail (as though you were still in a 50-50), turn just your hips and board 90 degrees into the boardslide. (It's all about ab flexion. Practice this shift on the hill first to get comfy with it.) This way, you turn into the boardslide while on the rail and then when your board leaves the rail, your upper body alignment will spin your board back straight for you before you hit the ground and keep you from wrecking on the exit. Effortless. From here, work up to jumping up and landing in the boardslide.
One more thing - when learning boardslide (50-50's too - pretty much anything with boxes/rails) start with boxes, then move to rails. Most people think they are easier to learn on.