i find that this one works great: 8-5-7-8-7-5 (repeat)
because it forces you to pick slowly in order to get it synchronised, and goes against the natural tendancy to play everything way too fast.
for changing strings, i suggest that you mute the strings with your left hand and practise picking groups of 2, 3, 4, 5, 6 notes up and down the strings, and alternate between starting on an upstroke and a downstroke.
good luck!