It might be helpful to know about the glycemic index. Here's a site that has a list of some foods (scroll down).
http://www.diabetesnet.com/diabetes_...emic_index.php
The glycemic index measures how fast a food raises your blood sugar.

The idea is to eat foods that don't raise your levels too quickly so you don't have the resulting sudden drop off.

String cheese comes in little packets that are convenient to stick in your purse or pocket for a quick snack. But the idea is to eat low glycemic foods every couple hours.

By the way, the book the site recommends by Jennie Brand-Miller is excellent. She's kind of a pioneer in the field.