Here's an attempt to summarize much of what I learned from others.

Green is the sheet diagonals that replace the ribs aft of the spar, alternating in a W pattern. Very effective in reducing twisting.

Red is the shear web that Tom described. Sheet infill between the top and bottom spars, grain is vertical. May only need webs in the first couple of bays. Very effective in reducing wing flex up and down.

Yellow shows gussets on the trailing edge/rib joints.

Blue shows bracing that runs from the top of one rib to the bottom of the adjacent rib. May help in reducing twist. Arrange in W pattern.

Pink shows bracing that runs in an X pattern in the bottom of each rib bay. May be combined with the blue bracing.

Not shown is something called a D Box which is a method of building up the leading edge to resist twisting. I thought the discussion of this was more appropriate for larger, RC type planes.

Looks great AK...

I think you'll find the gussets a great addition.  Sorry for not clarifying the "Shear Web" suggestion. 

For those who haven't Googled it yet=
Shear webs essentially turn a pair of spars into an I-beam. There is a reason why this shape is used for construction of buildings. It is light and strong. The webs add tremendous strength to the wing construction and help prevent a spar from compressing to the point of failure. (please note that the shear web has vertical grain between the spars for additional compressability strength.)
Pic below shows the shear web between the two spars, upper and lower spars.  Usually added to the wing beyond the fuselage out past the location of landing gear for support and strength and sometime clear out to the tip bay of the wing panels.

Not an engineer, but for the 3-D structure, in order to be effective, the angle from top of rib to bottom of the next rib would likely have to be at least 15 degrees to have any appreciable affect on stiffening. I just use an I-beam spar for that purpose. But the X-structure for the ribs adds a lot of stiffness.
ian

Here's a question for the structural engineers in the group!!

I'm considering adding some 1/16" stringers to a very basic wing structure, the idea is to 'stiffen' the wing a bit to resist twisting.

In the attached pic, the blue stinger runs from the top of one rib to the bottom of the adjacent rib (three dimensional). The green stringer shows a stringer running from the bottom of one rib to the bottom of another (two dimensional).

Does the 'three dimensional' arrangement provide more resistance to twisting than the 'two dimensional' arrangement?

Should the stringers be placed in a 'W' (sawtooth) arrangement, or should they be placed parallel to each other?

Or should I use some other design?