There's lot of good advice in this thread. Really invest yourself in those committed few when starting a program. Be positive, encouraging, and appeciative of their efforts. Make sure you get to know them well and are in tune with what motivates them. It doesn't matter whether they are studs or mid-packers. They will get better. More importantly if you do this right their friends will hear about what a great time they're having and your program will grow the next season or next year as those friends want to be a part of a good thing.
Remember that even slow kids have fast friends. Treat them all with respect. Make practices the best, most worthwhile, part of their day. They should be about a lot more than just running.
Incentives like t-shirts, over-night trips, etc. are ok but can eventually distract the kids from real achievement unless they feel like your program is a great investment of their time. Make sure there's eventually enough meat in the program to sustain real internal drive among returning athletes.
What excites you about coaching? For me, I love to see the sense of empowerment that comes from kids exceeding what they tought they could accomplish. Create lots of success opportunities on many levels.
Eventually you will have that experience like I had the 2nd or 3rd season of indoor track when school was cancelled due to air temps below -20 F. One of the seniors called and asked if we were going to have practice (roads were fine). I told him if he could get a group together, I would meet them for the hill workout that was planned. Twelve of the fifteen distnace guys showed up and pounded out a great workout in temps of ~-15F. 15+ years later and we have far more than half the team show up nearly every day all summer to runs/workouts organized by team leaders. They like to be together, to be part of something larger than themselves.
It's can become easy to expect a high level of commitment once it is part of the culture, but you can't forget to appreciate and to show your appreciation for their dedication/efforts. That's something I need to continually work on.
If you create a program that leaves kids feeling good about themselves and what they accomplish, it will grow and they will invest more of themselves. Be patient and keep doing the right things. Workout plans are necessary at some level, but they aren't going to be the reason your program matures.