Now, I'm not anywhere near as good as sweeps and arpeggios as anyone on here since I'm doing them at a very slow bpm.
One thing that I can't stress enough is that your left hand has to be prepared to haul some ass lol. So doing chromatics 4's or Rusty Cooley's "Legato Workout" is great to get all finger combinations going.
Now one thing that I'm noticed with the best sweepers out there is that they don't curve their hands to go up or down...They use a normal alternating up down without making the pick flow across the strings.
When I practise sweeps, I make sure I-
A- Pick as hard as possible, make sure you have pure resistence from the string
B- Hang on to a note as long as possible then letting go when changing string
C- Don't curve your picking hand so the pick is on a retarded angle
D- Practise them Super SLOWLY... I mean starting out on the lowest setting on your metronome and working your way up to a desireble speed.
Now most people will dissagree with me and say, you don't wanna strain yourself, don't wanna snap a string blah blah balh but if you play as hard as hell and have no-mercy with your pick, your gunna get 3 vital things- Clarity, Definition and Volume...All of which are essential to sweeps and arpeggios.
If you watch players like Jeff Loomis, Jason Becker, Frangk Gambale or Richie Kotzen, you notice they don't alter their right hand posture, and they are the best sweepers out there in terms of speed and clarity.
As for arpeggios, like everyone tells yta, arpeggios are notes of a chord played separately. That's true but the way I look at it is l;ooking at the main notes of a scale that harmonize with the root note within the key eg. C Major scale = C D E F G A B C high. Now If I were to play a C Major chord, the main notes/harmonizing notes would be C E G and High C. If it's minor you 'd obviously go flat 3 but still, all the same. It's just a matter of finding those notes on the fretboard and sonnetcing the dots.
Some people also add colour tones as well, to make sweeps/apeggios sound more "out of the box' or "exotic"... I will write up an example on this later on but it's best to learn from the guys mentioned above so scope around the internet or magazines to see what you can find.
Hope this helps