By any chance do you have a set of tiny screwdrivers that look like these: http://en.wikipedia.org/wiki/Image:Jeweler...-set-bright.jpg (or similar)? I'd take the smallest one and see if it can fit in a tri-wing screw and try to lock it inside and unscrew the screw. I've tested it on my old Game Boy Pocket and it seems to work. I think the DS Lite screws are too small though so this might not be possible and you might also strip a screw doing this. I wouldn't recommend this method, just get a tri-wing screwdriver if you can. Without using a screwdriver, I don't know of any safe method of taking apart the Lite.
I used a very small flathead screwdriver. One that fit into the tri-wing grooves, of course. I then pushed down on the top of the screwdriver with my palm and took pliers to twist the screwdriver. I also, without the pliers, just turned the DS instead of the screwdriver. If you put enough force on to the screwdriver, it will work.