Bleach longer, use a good dark blue. Special Effects has an awesome really dark blue that almost looks purple when you first apply it. The dark blues are good because after a few washes they are a nice bright blue and stay that way for a bit. Another thing I do is put a bit of the dye in my shampoo so when I wash my hair it doesn't strip the color so drastically. And my blue hair looks pretty effing rad if I do say so myself
