The best method I've seen is Nick Schade's 2x2 inch (or is it bigger?) square aluminum tube.
I used a thin wall 2 x 2" square steel tube with 4" wide 1/2" Baltic Birch strips screwed to either side (vertical to keel) with self tapping
screws. Very heavy, but very straight. (the steel cost about $18, aluminum would have cost about $100)
I attached the plywood so I could more quickly screw on spacers with drywall screws.

I also copied Nick's spacers. These aligned each station quickly and accurately. There was some fudging in the middle to account for the stations running inside the line from each end.

Ends are very critical as Jay points out. I used blocking (1/2" ply 90 degrees to forms (horizontal to keel) on either side of the ends to ensure that they were straight, true and stiff.
Then when I applied the first strips along the shear I again made sure I didn't pull it out of alignment. After this it's pretty bullet proof.

On my first build I ripped 1/2" Baltic birch ply (I used to get 5 x5' sheets for $22) 4" strips, rabbeted the edges.
I staggered the joints. I couldn't have made a bigger headache if I'd hit myself in the head. The strips were good, but the staggering created an unsolvable problem. (I fixed it by clamping it to a piece of steel, then by wedging and tweaking each station using string and a laser.)
I think making a series of straight plywood boxes and then aligning the sections (attached to an internal piece of 2x or 1x) would work.

Most methods I've seen beginners use, and those using single 2x4s create more problems then they solve. (House framing is a process of using 2x lumber that will continue (or want to continue) to move and twist for years and years, in webs that are made straight, true and flat by multiple members resisting each other. 'Problems' are worked out with a sledge hammer-and then nailed down. The sheathing material finally locks it all together. You will never see a single 2x4 used to create a straight unmoving element.
If a carpenter were to make a strong back using 2x material he would use at least three elements. Maybe 2 2x4s and a 1x. More likely he (or she I guess) would build a box beam, 4 pieces with the two connecting pieces inset from the edge--so the edges could be planed straight.