A good light.
Outdoor Portrait Photography can be the most rewarding when you use the right lighting. The two photography tips I am encouraging you to focus on first are finding a good location where the lighting is good on your subjects and choosing an angle where there is good lighting on your background as well.
Keep I mind as the seasons change so does the light for your photography session. Natural light can make some of the best and most beautiful photos, however sometimes its not enough. Often as photographers we know that shooting in mid day is the worst time of day to be shooting as the sun is at its highest and can create bad shadows on your clients faces. A good photographer can make any situation work if you have the right eye. If its cloudy out then shooting at high noon is not a problem, however if the only time your client has is during the middle of the day and they want out door shots you have to improvise. Try to talk them into a place that has shaded spots and use the shade to your advantage! If you are doing just a few people are just one make sure to have a reflector they are relevantly cheap and can help you in so many situations.
In my opinion the best time is in the afternoon but of course this will depend on the time of year too. Also early morning is a great time for sunrise. Photo opportunities always present themselves at different times of day and having a few tricks up your sleeve can help you master even difficult lighting situations.