In 1999 we made a scissors lift, and it actually worked quite well.
The members were made of marine grade 1/2" plywood
we used aluminum rods to hold its stiffness in plane.
we did one thing very differently than many others to get success with this mechanism. Typically people use some sort of linear motion to "pull" the bottom member and thus cause the raisin scissor action. Instead of this we used a cable and a pulley to lift the middle joint of the X part of the bottom member. We also never let the mechanism collapse all the way down, keeping the angle between each X stage more manageable.
I think I did a horrible job of expalining that, If you are interested in a better explanation, PM me and I will try to get you some pictures.
The only picture I could find is this:
http://www.firstrobotics.net/99Galle.../121-1_jpg.htm
There are more pics on
www.rhodewarrior.org, but it is down right now because of bandwidth problems
Hope this was helpful.
PS...the lift we made lifted a load of about 15 pounds and the top of the lift reached over 10 feet.