I would suggest keeping an eye out for times when conditions aren't optimal -- and then go practice during those times. Try different things to find out what works in those scenarios so that you are prepared when you have a commissioned session.
There are a number of things that contribute to making conditions less than ideal. Here's just a couple and some suggestions of what to do in those situations. Keep in mind these are very generic and each situation is different, so keep your eyes open and be creative when trying to make the most out of these situations.
1) Windy - Position your subjects so the wind is blowing into their faces rather than from behind. Also, look for areas that are natural wind barriers -- tree lines, buildings, etc. to minimize the wind. You can also use the wind to your advantage to capture long hair gently blowing in the wind just right.
2) Harsh overhead light - Find some good open shade to place them in. Or use supplemental lighting to help balance the ambient light. (Off camera flash, not pop-up flash!) A polarizing filter can help in some situations as well. In harsh light you can also have your subject face away from the sun to backlight them.
3) Rain - Get some umbrellas and have some fun! There's not much you can do here if your subjects aren't willing to get wet Actually a good soaking can help to bring out colors and contrast in your images. For example, tree bark gets darker, making the green leaves stand out more against them. Same goes for streets -- they get darker and absorb more light instead of reflecting it. You can also utilize puddles for jumping in, reflections, etc.
This is just a couple quick thoughts to help get you started. The bottom line is that the more you practice in these situations, the more comfortable you will feel about what to do with clients when you encounter these conditions. Part of being a photographer is understanding how to adapt and create good images even when conditions aren't perfect.