I agree Abby Whiteside's take on arps is most helpful!!!

Pete

I think a good way to prepare for arpeggios (which are more difficult than scales) is to play 4 note broken chords slowly and clearly, using the metronome at a slox tempo, say crotchet (quarter note) = 40 (or even slower). Play with precise downward motion of each finger. Then when you feel comfortable with the tempo and can play in time, increase the metronome speed a few notches. Over a period you will become more fluent. My experience is that this is beneficial with playing arpeggios. By the way don't strain your thumb crossing it under the fingers when playing arpeggios is an effort to get a perfect legato. With many hand sizes it's just not possible.

I agree with romanza's post on this one... I didn't read past that post, so I don't know what was said after it.

Except, instead of playing them as triplets, play them as 16ths. This helps to make them not "lumpy" and makes them a lot more smooth.

Also, just a minor difference, but instead of bumping up 8 or 12 bpm each time, I only do 4 (I practice everything this way... start out at half tempo or slower, and then once it's perfect at that tempo, bump up the metronome 4 bpm. I guess the 4 bpm thing only works if you have a digital metronome come to think of it... if you don't have one though, you should go out and get one anyway )