supplementation

I am competing in a competition called a Pump and Run. It involves benching your bodyweight a maximum number of times and then running a 5-K. My weight is 155 and I want to keep it that low so I can have the benefit of benching less weight. I also have to improve my strength endurance. I can currently do 155 18 times. Do any of you guys have supplementation suggestions that might help me improve my strength endurance, while not adding any extra bodyweight.