See Julian's post immediately below for cases with a still-long but not long-enough steerer tube, or where only a modest height increment is required.
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Here's a simple introduction to raising your bike's handlebars even if the steerer tube has been cut short. In each case you need extra ironware.
1. A modest height adjustment by an external clamp-on extender, generally about 40mm max, which I found not enough:
https://www.sjscycles.co.uk/stems/55mm-zoom-headsup-stem-extender-clampon-steerer-extender-for-ahead-system-forks/ The one of this design I had was the BBB BHP-22 but they're all basically the same. I suspect they're all made in the same Taiwan factory (Uno, used to be called Kalloy) with the more expensively branded ones maybe finished a little better. I lent mine to a lady pedalpal who shamelessly refuses to return it. I've taken her recipe book hostage...
2. A variable height adjustment by an insert which is fixed by an invisible sliding, expanding wedge-clamp, which is what I currently have on my bike and which does very well, thank you:
http://www.wiggle.com/bbb-bhp-2021-stem-extender/ My current one came in an n'lock stem kit, made with Swiss punctilio, but the BBB BHP-20/21 is basically the same thing, mass-produced. I wouldn't be without it for any money.
I mention the BBB model numbers because they're generally in stock at the bigger LBS, or almost instantly available in the next regular delivery from their pet wholesaler.
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Here's a generic photo of the two types and three general shapes of steering extenders from the sainted Sheldon's pages at:
http://sheldonbrown.com/handsup.html***
To order the right component you have to match the size of your steerer tube ("for 1-1/8in or 1in steerer tube") and the size of your stem (ditto). It's not rocket science but if you're uncertain, ask here with such specific details as you know and someone will help you sort it. Don't busk it: wrongly fitted steering components are a face-plant waiting to happen.
