Don't say practice. I practice two to three hours every day. For alternate picking specifically i do the chromatic scale two ways:

1) start on low E and go 1-2-3-4, move to A 0-1-2-3-4, go all the way up to high E
2) On high E i go: 1-2-3-4, shift, 5-6-7-8, shift, 9-10-11-12 and back down.

I make sure EVERY note is alternate picked and no hammers or pulls. My goal is to be able do swiftly and accurately do two groups of 16th triplets a'la Metallica's One. Any other excercises i can do?
don't just do chromatics. I highly reccomend Paul Gilbert's Intense Rock videos. They are very helpful in getting accurate and fast alternate picking, as well as some sweeping and string-skipping.
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Chromatics are great for finger independence, but the only thing they'll help you do speed-wise is play fast chromatics. Learn difficult songs that you enjoy, learn licks, do something musical.
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