There are many drills that have been created to improve your Ultimate skills. It is not important to know a lot of drills. What is important it to have a few drills that your team all knows and executes well. Drills should get everybody touching the disc as well as have everybody doing all aspects of the drill. Work on developing everybody as a complete player: defender, receiver, and thrower.
When choosing a drill for your team, first decide what you want to work on. Second, decide how much the drill should involve conditioning. Drills for beginners should focus more on the skills and less on conditioning. More intermediate drills should focus on executing those skills under pressure and/or while tired. As your team develops an offense or a defense, adopt or develop a more advanced drill. This more advanced drill should focus on particular aspects that are unique to your offense.
Additional Web Sites of Interest
The Ultimate Handbook
Video Papers by Dan Cogan-Drew
PlaySports Instructional Clips