Science gateways require a wide range of considerations to make them successful and sustainable. One important consideration is usability. Gateway creators typically lack usability expertise and do not know how to avoid common usability problems. The Science Gateways Community Institute (SGCI) has provided numerous workshops, webinars, and consulting services to the community to improve gateway usability. Based on more than 30 in-depth usability evaluations conducted over the past three years, we identify a small set of common usability problems that gateways have. We characterize these problems and present some possible solutions. Additionally, we show examples of problems and solutions from gateways we have worked with. This paper can help gateways creators identify common usability problems along with solutions that are easy for non-experts to implement.