Jakob Nielsen's 10 general principles for interaction design. They are called "heuristics" because they are broad rules of thumb and not specific usability guidelines.
Before you go hire a business coach or sales person, enrol in an online course, follow a get rich quick scheme, do this first. Warning, this is a long list of things to do. No easy answers, no quick fixes, but the bare minimum of what you need to do before looking for more help. Remember, people hire who they know, who they like and who they trust. So time to get known.
Follow proven formulas to land more clients and simplify your proposal-writing process. Tested in the real world. Yours for free.
Anyone including non-designers can use this framework to analyse a product or service or analyse a piece of work for a potential client. You could even use this to review your own work as the tools and techniques I use leave little room for bias.