(1) Choosing the right spring. The spring must be light enough so that the class can see it stretch out.
(2) Scraping the track. If the spring attaching the cart and the end of the track lies along the track, it will scrape. This can be reduced by having it scrape over a smooth piece of paper or laying down some scotch tape on the track where it comes in contact. Another method is to suspend the spring, either by balancing it on a rolling device or attaching it pendulum-like by attaching strings to the coils from an overhead attachment such as a crossbar on a ring stand.
(3) Any such method in (2) should be tested before class begins to ensure that the particular arrangement doesn't result in large error in any calculations the class might conceivably do. Improper suspension of the spring or an excessively massive spring will affect the quantitative results.