@sadieme, when you shift gears you do what is called "light" pedaling, meaning you keep pedaling but you decrease the pressure you are putting on the pedals. A good thing to know is that as you see a hill coming, don't wait until you're halfway up the hill and realize it's too hard to keep pedaling, but ANTICIPATE that the hill will be more challenging and shift to a lower/easier gear just as you begin to climb. You can still shift gears when you are pedaling strongly, but the increased load on the chain while shifting is not optimal. The idea, in general, with pedaling is to get into a comfortable & sustainable cadence (revolutions per minute) and be able to maintain that whether on the flats, going up or down a hill. So going up a hill you'll need a lower gear, and since going down a hill is easier due to gravity, you would shift to a higher/harder gear. There is a trick to shifting if you're already going up a hill and you're pushing down on the pedals really hard: you slightly turn to the side and give 1 hard pedal, and because you're turning perpendicular to the hill it takes pressure off the chain for a split second and that's when you shift down to a lower gear. It's easier if you see it vs reading about it.
Regarding your saddle (seat, but call it a "saddle"), if you are sliding forward I would try slightly tipping the nose of the saddle up. My saddle uses a 5mm Allen wrench with 1 bolt attaching the clamp to the rails beneath the saddle. I loosen up this bolt (not all the way, but enough so you can tip the nose up or down), and as I move the nose I hear and feel "clicks" and I move the nose only 1 or 2 clicks, tighten and go for a ride. So etimes moving a saddle too far forward will cause you to put more pressure on your hands resulting in sore hands, wrists, shoulders.
Also check your tire pressure daily prior to each ride. The pressure (usually a range) is listed in the side of your tire. Using a lower pressure in the range makes for a softer ride. Tubes naturally can loose 3 or so psi per day.
