BEst thing you can do is to practice drumming your fingers back and forward, a) 1234, 4321, 1234, 4321
b) 1234321, 1234321, 1234321
c) 1234321234321234321234321

THen to get things difficult and prep you for sweep picking...
a) 1324, 4231, 1324, 4231
b) 1432, 4123, 1432, 4123
etc, you get the idea?
Just mix it up in whatever mind bending way you can. You will find some combinations almost impossible the first few times but the more you do them the easier they get. The above really helps for sweep picking because of the arpeggio shapes.

I also pick along with my right hand when I listen to music, it helps you to focus on timing without having to worry about the fretting hand or actually playing anything.
You can do those exercises in work or even on the steering wheel when you drive. The picking exercise is not really about physically picking but more about understanding rhythm and can be also done by tapping your feet or fingers - no matter how many times you try playing something you won't get it right unless you really understand the rhythm behind it. When you understand the up/down stroke patterns and inflections (very easy to work out using your feet or fingers like a double bass pedal 1,2,1,2,1,2) then actually picking them becomes a lot easier.

Sounds a bit 'out there' but it works very well for me, hope that helps.