The first step to making your partner look good, is to make your partner feel good.
While it is certainly possible to perform great improv with scene partners your hardly know or have even met before, it is utterly impossible to improvise together with someone who doesn't make you feel good.
With strangers you can have a good vibe, a great first impression, a wonderful connexion. But with people you have worked with many times over many years in many productions, you can reach that next level of connexion, leading to simply better improv.
Go out of your way making your partners feel good, either physically with a quick massage before a show, or emotionally with ample attention, compliments or enthusiasm. If your partner feels good, they will look good. And that's our goal after all, isn't it?