Are you referring to dry land exercises or water? Also, what are the distances you are trying to compete at/complete? What is the stroke you are trying to improve? Are you trying to improve your stamina or your speed? All of the above questions are important when it comes to determining the best exercises to improve swimming technique. The exercises Joanne and Calvin mentioned are very helpful, but I would also agree that hiring a swimming coach would be a great idea as well. If your technique needs to be improved, the coach would be the best person to help you since he/she can observe you in the water.