I've had a fair few shows where my booth is open on two sides and there's two things I found that have worked for me...
1) I embraced the fact that "the walkthrough" is inevitable and used it to my advantage. If you have more than one entrance, no matter how small, there will -always- be someone who uses it to just walk through. People walking through, however, catches others peoples eye and for a split second they might look like a shopper, which we all know draws other shoppers in. So instead of cursing the somewhat rude walk-througher, I thank them. After mentally cursing them a little, of course. :P
2) That being said, I don't want to make it easy for them to just walk through all they want. So I stagger the entrances by placing either a table or clothing rack in front on opposite corners, blocking about half the booth space, making the person have to weave their way through. (With the rest of my products along the sides.) At times I have even put a smaller display rack right in the middle, to make them weave even more. I've noticed that when someone has done this once they wont do it again because it's not as much of a shortcut they thought it was. :P
Personally it's worked out pretty darn well every time I've had a front and back entrance to my booth. Just gotta embrace and innovate! :D