Okay, I'm feeling ridiculously lazy right now, so a lot of this is what I wrote to someone else in another thread about distance running :)
Before a run, your BS should be at least 140, but lower than 250. If you're going to run within 2 hours of eating, then you should only take half of the insulin that you would normally take. (This varies a bit for some people).
If you're running in the mornings, you'll have some insulin resistance (known as the "dawn phenomenon"), so you have to eat and take insulin before working out. Later in the day, you'll have less resistance, and will probably requre less insulin.
This probably varies a bit from person to person as well, but I noticed that by blood sugar would drop during very slow jogging. It would raise a bit during harder runs (150s-160s), but be lower after my workouts. It would raise more during speed workouts (170s-190s) and be lower afterwards. During races, my blood sugar would sky rocket (260s at the end of the race, always), and be a lot lower later. So basically, slow running lowers blood sugar during the workout, but not as much afterwards. High intensity running raises blood sugar during the run, but makes it a lot lower later. You'll also notice that after running consistently, your blood sugars will be lower all day so you should gradually reduce your long-acting insulin (or change your basal rate, if you're on a pump, for days that you are running, for days that you're not running, and for during your runs).
Also, you should always glucose tablets with you when you're running, and a medical bracelet is really great to have in case you pass out and someone finds you. It's also really great to have someone to run with, especially at first when you're testing out different doses and snacks.
I carry my glucose tablets in this: www.amphipod.com/.../airflow-microstretch-belt
It's really small (but stretches to 4x it's size) and you can't see it under most clothes.