When you superman (forwards) you want to be as close to the ground as possible.. you want your torso/arms as parallel to the ground as you can. I usually find this easier if I try and imitate a "roadie run", or crouch run. Once you get to that you want to push with your last foot that touches the floor, and most importantly, push your arm's through! If you don't push your arms you'll land on them/your gun/do a front flip and seriously hurt yourself!
For baseball slides it's also about getting low, but more about momentum.. You want to get as much force behind you in as short a time as possible. You also want to try and use your outside foot as a pivot, so that when you are at your destination (preferably behind a bunker!) you can dig your outside foot in and snap up on to your knees/crouched - This'll reduce the time you spend not shooting and allows you to conveniently snap shoot at a different height level!
One of the best things you can do to work on your sliding is give yourself a set run-up distance of a meter or two, and have a friend kneel with his arm out. This will mean you can see how much momentum you need to carry you past them and work on staying as low as possible, to avoid getting shot!
TL;DR - Momentum, stay low, practice and more momentum!
