I've been playing for almost three years now and I want to be able to sweep. Can anyone help with some techniques or exercises?

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well ill start off by just saying that you have a sick fortress icon.

and i would say the easiest sweep in the book is the 3 string sweep played in the beginning of the song bleeding mascara by atreyu. you might hate them, w/e i dont like them either but this is what started me sweeping and is a good place to start. the key to sweeping is practice, practice, and more practice. it sucks to learn but after a while it will begin to come and you will continue to improve. its a lot of fun to do as well.

you can find a tab for this on UG, search for the song and click on the intro, its the second one down.
repetition.... repetition repetition repetition. seriously its all about repetition. you really need to have your hands synched pretty good to do it fluidly. i've been practicing it for about a year or so and just the other day had a "breakthrough" where it all start to kinda fall together. im sure if you practiced it for more than 10 minutes a week you'd prolly catch on quicker than i have though.
don't play this dumb band... hammett also did 3 string sweepings on creeping death and leper messiah
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There are some excellent help articles on UG and the rest of the net. There is only so much someone can post here that would actually help you that much.

The key to sweeping is time. Slow the piece down to a very steady pace and try and practise with a steady beat - drummer/metronome/drum machine/foot tapping.
If you are totally new to the art of economy picking (sweeping) and the 'rolling' of your fingers along the strings, no matter how good you are, sweeping will be a nightmare.
Accuracy is also clearly important and rolling your fingers across the strings with a combination of down and up sweeping motions will get you on your way.
Actually examples are very difficult to post here.