Three Tips
It is a good idea to have memorized ahead of time some appropriate quotes which can be applied to almost any situation. Using them effectively will not only start your table topics speech off on a reasonably strong note, but it will also give you some time to think and develop your topic more fully.
Don't feel that you need to start speaking as soon as you are introduced. (If you do, it automatically starts the clock, and you are committed). It's fine to wait a few seconds (some recommend as many as 10) to collect your thoughts before responding.
Remember that strong delivery and gestures can make the difference, particularly in higher level contests. I'd suggest that you continually volunteer to participate in Table Topics at each meeting when preparing for a contest, and even ask your friends to toss you some Table Topics questions outside of meetings to practice responding.