Hey mate.
I have had Type 1 diabetes for 15 years and have been mountain biking for about 4 years.
First off, I believe red_dog was talking about a friend that has an insulin pump, which probably makes things a bit easier.
However, it doesn't sound like you do. I take 4 injections per day. What I do to avoid hypos is to:
* if you are training for an event and know that you are going for a ride, reduce the insulin dosage (this is easier if you are taking four injections a day and are on Lantus - a stable 24/hr long acting insulin) before you go for that ride.
*eat some energy food or drink lemonade before you go on a ride.
*if it's your thing, eat during a ride. I'm not a big fan.
*eat some low GI carbs when you get back.
Obviously all these points depend on how intense and long the exercise is. My rides only last for 1 to 2 hours, but my average heart rate usually is >70% and can average over 80%.
During 12 and 24 hour events, I often won't take insulin until the later stages of the race, only small amounts before the race, and I eat heaps. It's great, you can feel like a person with a normal pancreas!
Exercise can also affect blood sugar levels and therefore insulin doses after exercise, even the next day.
Admittedly, I do hypo sometimes after exercise. You have to find out what works for you. One point that you must stick to is - always take some high energy food (liquid is best) and some carbohydrates when riding, to guard against an unexpected hypo on the trail. It may happen and you don't want to be in the middle of nowhere and hypo and have the only option of riding out and having a low at the same time. It sucks!
When I was 16, I was basically a meat-sack full of hormones! This can screw with glucose levels quite a bit, so all I can say is, just stick with it and don't get too frustrated as things get better and there is only so much you can do.
You have probably heard this all before and my last point is, which you probably also know, there are plenty of elite athletes with type 1 diabetes. Oh yeah, you might of seen a letter in the Dec/Jan issue of AMB that a diabetic wrote.
All the best.