3 notes per string all across and back over the strings is an excellent way, Paul Gilbert recommends it aswell

you're asking about economy picking, not just plain alternate, right?

For economy picking you alternate pick when playing more than one note on a string, but when changing strings you Always use a pick stroke to take you directly to the next string.

so to economy pick bluestrats example you would pick the three notes on the low e string down, up, down - then when you change to the a string you would also pick down, up, down. this way you you are picking directly to the a string.

in strict alternate picking you would pick the e straing as i said and then you would jump over the a string so that you could pick its notes as up, down, up.

i recommend using 3 note per string shapes to practice economy picking. when you want to turn aroung to come back you just play an even number of notes on that string. then when you are coming down the scale all string would be picked with an upstroke first.

that's how economy picking works. some people who are very used to strict alternate picking don't like economy picking but to me it is more natural.