The trick with Sneaking is in the footwork. What you need to do is roll the heel to the toe, making sure the center part of the foot does not touch the ground (hint: this does not work on gravel or leaves; see below).
Try to find areas with living, not dying, grass for outside, and for inside stick to carpeted areas and avoid any objects that leave a print (i.e. dragging your foot on shaggy carpets leaves a streak).
For areas with gravel or leaves, you must stay flat footed, pressing so that all the weight on your feet hits the ground at once. This takes practice, and must be mastered before using it, because even masters of it cannot stay completely silent on gravel. In areas with leaves this works well too, but the best solution is to avoid it. Try looking for vents to climb into and around the gravel or leaves, otherwise, find a way to climb over it (i.e. pipes, beams, etc.).
For climbing, try to find pipes or beams that are spaced from the wall, so you can grab the other side and put your feet on the wall. Most people find climbing difficult, but the easiest way is with your feet on the wall. Apply some pressure with your feet on the wall, and exert pressure with your hands on the pipe or beam. Always check what you are climbing by jiggling the pipe or beam, make sure the climbing fixture is well bolted or cemented into the structure, and slowly climb it so if your weight does break it you will be closer to the ground. Also this makes you quieter.
When hiding from guards or people you are avoiding, find a place where there is a gap (i.e. if there is a space in between furniture or baskets, squeeze into the area). Once you are in the area, press yourself against the wall. If they are going to be close, learn to control your breathing. you can either breath in short hot breaths, or elongated, cool breaths. Test each of these on your hand before trying either. Make sure you can leap out and grab the guard if he notices you, but stay still, because chances are, he wont. If he does, however, be sure to grab whatever weapon he has. Another good way to hide is on beds, if you manage to sneek into a room with a bed, but you hear someone coming, jump underneath the covers and make yourself as flat as possible. Once again, learn to control your breathing, however you are laying, make sure the way your stomach pushes out is sideways of how you are laying (i.e. if you are laying on your back learn to make your abs push your stomach to your sides). This does not work if the bed is fresh made and the sheet are taut against the matress. Try to find a bed with messed up sheets and pillows in the middle.Only use these skill if you plan them with a friend, do not use them to break into houses and steal stuff or other illegal things.