I've painted a lot of walls & ceilings with rollers (rental properties).
I just recently came up with a technique that makes it faster &
easier. I use a dishpan to hold the paint, with a roller screen in it.
The roller screen I mean is the one made for painting directly from a
5 gal bucket of paint.

http://home.comcast.net/~bobengelhardt/PaintTray.jpg

I also put the dishpan in a regular roller tray to tip it up & bring the
paint to the front.

The advantages:
- it holds a lot of paint. As much as a gallon. So refilling time is
cut down, if not out.
- it's more secure. Paint's not as likely to slop over the edge
- the screen works better than the solid surface of a tray.
- it's a better angle than having the screen in a 5 gal bucket, when
you're using an extension handle.

I also just discovered lambs wool roller covers. They put a lot more
paint on the wall, per dip. About twice as much as the synthetic covers
that I had been using. They don't seem to hold more paint, rather
release more. I.e., the synthetics seem to keep more paint in the
cover. Lambs wool is lot a more expensive (about $8), but well worth it.

Bob