How to get that 'soft light' portrait look when taking pictures?

I'm taking senior pictures for a friend of mine tomorrow. I have some basic photography knowledge and experience. It's going to be mostly/all cloudy tomorrow. We're going out at about 8 (sunrise is at 7:50) and we're taking in a few various locations. We should be done around noon or so.
I'm taking them with a Nikon D90, most likely the kit lens (18-105mm, f/3.5-5.6G ED VR), and may or may not have access to a speedlight. The photos will be taken outside.
How do I get the lighting right to ensure that aesthetically appealing 'soft' look to the photos?

You need to soften the light using some kind of light modification device. This can be an umbrella, softbox or bounced lighting.

That is really something that only experience can show you. Shooting under noon lighting is perhaps the WORST lighting.

Instead shoot in the morning or late afternoon. Shooting in the shade with a flash fill may just do the job for you.

Here is a sample of a shot taken just at sunset with the model in the shade.

It's called a soft focus lens.