I used to suffer with stitches when I first started to run. I found slowing down my pace really helped. There is also some breathing technique as well but never quite got the hang of it and some people suggested running with your arms out like an aeroplane.
Preventing them is a little tricky; the stitch is essentially caused by the contraction of your intercostal muscles so trying to ensure a good flow of oxygenated blood to those muscles is the key. I usually get my stitches from increasing the demand on my lungs like running up a hill or increasing my speed without adapting my breathing properly, so that's something to think about.
Once its too late though; I find slowing down a little bit so that you can take deeper breaths helps - you can then 'send' the oxygen down to your intercostal muscles more effectively. Giving the intercostals (around the ribs) a little massage (just give them a squeeze and a prod as you're running along) also helps disperse the sensation as you're effectively promoting fresh blood flow to the muscles and decontracting them. Grabbing and/or squeezing and holding the waist can also help.