I have been running almost exclusively treadmill for about 13 years. I also did a number of road races. I found some tricks to be very useful to let me move between the treadmill and the road.
Learn your body while on the treadmill
First, pay attention to how fast your legs are moving on the treadmill. Vary the speed, notice how the legs change. Increase the incline, same pace, notice how you have to work harder even though your legs have the same cadence. Get a heart rate monitor and you can also compare sensations with specific numbers on the HRM. Treadmills definitely have their place, but they require more conscious effort to learn.
Do outside runs before the race
If you want to run races, you might want to do at least one or two outdoors runs shortly before the race. This gives you a chance to adapt what you've learned before you actually need it. If you can, best to use the actual race course.
Watch out for uneven pavement! The treadmill will let you get away with not paying attention to the surface. Actual roads and sidewalks are less forgiving. Cars, bicycles, and dogs even less so.
Also, going outside means dressing for the weather. Remember that running produces a lot of your own heat. Dressing appropriately can be tricky. For me, assume the body will warm by about 20 degrees fahrenheit.
Incorporate what you learned back on the treadmill
After a period of road racing, when you go back to the treadmill, adjust your incline until you roughly match your course speed. That gives you a feel for how to translate back and forth. Especially useful if you will redo the same course in a future race or year.
