RC slope soaring is made possible because of the action of the wind. Any moving air that encounters rising land is naturally forced upwards over that land, and this upward movement of the air is called ridge lift. It's this rising air that you're using to keep the glider airborne - if the wind stops, or dies down, then the glider will only stay up for so long.
The optimum area for catching this ridge lift is just in front of and above the upper reaches of the slope. If the wind is constant and you keep your glider within this area, you can stay slope soaring all day long!
So to get the best out of it, you need to fly your glider parallel to the slope face and not too far away in front of you i.e. fly by making a pass left to right (or right to left), turn the glider through 180 degrees and repeat the pass going the other way. The important thing to remember here is to always make your end turns into wind i.e. away from the slope. If you fly the glider too far away from the slope face then you risk flying out of the rising air and losing lift, which can result in a long walk down the slope to retrieve your fallen glider!
As you fly from side to side, you'll notice that your glider will gain altitude with each pass - hopefully. This is the rising air pushing the glider upwards. If you're confident to do some basic aerobatics, just let the wind carry the glider higher as you fly it from side to side. When you've reached a good height, now's the time to have some fun!