Students commonly have difficulty with "synthesis problems", which require a combination of typically two concepts that are taught separately in different chapters and/or at significantly different times during a course. One reason for this is that students frequently rely on a formula-based approach, beginning by searching for mathematical equations or worked examples which often do not exist. We employed a guided scaffolding method to induce students to employ a more effective problem-solving approach by first searching for fundamental concepts. This method includes a sequence of two conceptually-based multiple-choice questions that have similar deep structure as the synthesis problem, and an explicit instruction to remind students to make connections between the synthesis problem and these conceptual questions. We report our findings on the sustained effects of repeated training using conceptually-scaffolded synthesis problems. In the last 2 weeks of the 2009 fall quarter, we repeatedly provided 3 groups of students with different training using scaffolded synthesis problems, un-scaffolded synthesis problems, or traditional textbook problems. Four days after the training, all students took a common final examination containing a synthesis problem without scaffolding. Results show that repeated training with scaffolded synthesis problems rendered the highest success in students' correctly identifying and applying fundamental concepts for solving this problem.