Mainly it is the length of your fingernails - you'll see many of the fingerstyle guitarists (andy mckee, antoine dufour, ewan dobson, daniel voth etc) have false nails. If you play enough, your own nails will actually wear down faster than they can grow!
Other than nail length, it's the string attack (in much the same way as you alter the slope of your pic) and how hard you hit the string that you use as emphasis on certain notes.
Also, where on the string you pick makes a huge difference - pluck the strings close to the bridge and you get a much brighter sound where the emphasis is on the 'treble' frequencies (you may find faster picking easier here as the strings have less flex). Pluck closer to the fretboard and you get a deeper warmer and more bassy sound.
A good picking exercise is to play a chord and use all 4 fingers and your thumb to pick each string in turn, up and down the strings, letting each note sound out clearly. Once you have that in the bag, try and pick one note harder than the others out of the sequence - this will teach you to be precise with the thumb/finger sounding that string/note louder.
Pro tip - when picking, use an 'anchor point' to stop your hand flailing about and causing your fingers to miss the strings.
- when using 4 fingers & thumb picking, try and use your elbow joint on the hip of the guitar. (you can use the heel of your palm against the bridge too, this helps when you want to do right hand string muting)
- when picking with 3 fingers & thumb (so thumb, index, middle, ring finger) you can use your little finger as a rest. Imagine you are posh and are drinking a cup of tea

I find planting the little finger somewhere near the bridge/pick guard area to be best. It allows you to move your hand over the strings with a finer degree of control.