Disclaimer - I am not an expert at off camera flash yet. I am still learning :)
Off camera flash is a useful skill to master. It means you can add depth and contrast using flash. If you stick to the on camera flash, a lot of portraits will be very flat lit. The contours of a person's face will be lost.
Below is an example of when I was practising with off camera flash at night. I positioned the light at a 45 degree angle to my friend in the first photo, and had two light in the second - a bright one for a rim light, and a fill light to light her face. They're not the best examples, but as I said, I'm still learning!
After seeing the demonstration at The Photography Show in March, I'm going to experiment with the one light through an umbrella to see how it comes out. I have a much better understanding of my camera and settings now.
So, for some actual tips!
+ Use a flash meter! These range from £150 ish to top of the range ones such as Sekonic that cost over £500. I'm saving up to invest in one.
+ Invest in a TTL flash. TTL means Through The Lens, which means it registers the shutter speed, aperture, and ISO settings and automatically puts itself to the correct setting. You will need a wireless flash trigger and remote that is compatible with TTL. I really want Cactus RF60X flashes as they act as their own receivers and transmitters, or the PIKA200 by Pixapro. That one is powerful and small.
+ Experiment with angles. 45 degrees is a good start, but have a go at placing the flash higher than the model, or to the back to make a rim light, or below to cast strange shadows.
+ Make sure you have plenty of batteries