(Fig.1) Make you mark..
Some bars come with cutting marks printed on them. If not use some tape, a narrow permenant marker or maybe a pencil depending on surface.
Remember: Don't cut off too much in one go. Shortening your bars can seriously effect your riding style/stability. Too little is better than too much.

(Fig.2) The Guides..
I wanted to use a cutting guide of some sort to ensure a nice straight cut, so had a rummage through me toolbox. There I found a couple of old grip-clamps ('Lockons' from ODI grips) lying about, which would make the perfect (well.. steel would have been better actually) guide.

(Fig.3) Lock-on your Lockons..
For my bars I chose to remove only 30mm from each side. The inner guide should probably sit about half a MM to the inside of your mark if its thin. On the WingBars the marks are already quite wide so I just butted the guide to the edge of them. See (A) on large image (click thumbnail image).

(Fig.4) One ring or two?
Its quite possible to get a nice straight edge on a cut with only one guide, but as I had two I decided to use both. If you do this remember to leave enough of a gap for the flaring of the teeth on the hacksaw. These are only guides, you don't want the teeth of the blade to hit them too much.

(Fig.5) Push or pull?
ToolTip: As far as I know, conventional hand-held Hacksaw blades are always directional. In other words they do all their cutting on either the forward or back-stroke. Its pretty easy to tell which direction the blade is going to cut best in, most of them even have it printed on them with a big arrow.
In the picture on the left, the blades cutting direction (as it lies) is leftwards. Its not a wood saw and shouldn't be used like one, you should take care not to press down on the blade when its on its 'non-cutting' stroke. Doing that will only result in blunter blades and rounded off teeth.
Interestingly (maybe not), western blades were traditionally designed to cut on the forward-stroke and japanese blades on the back-stroke. Some boffins say you get a smoother but slower cut if you cut on the back-stroke. I decided to turn my blade around to try it.

(Fig.6) Easier cutting..
Whenever your cutting metal its good to use a little lube of some sort to stop the blade clogging with chips and keep the blade gliding smoothly. Ideally I'd have had a proper tapping lube, but all I had to hand was teflon penetrating lube so used that. Thinner oil is best, apparently baby-oil works very well (untested by me)

(Fig.7) First cut..
There is NO need to race through the cut. As our Metalwork teacher used to say, 'Let the saw do the cutting'. Apply a moderate amount of pressure on the cutting stroke and none on the reverse stroke. Cutting fast and hard only heats the metal and evaporates the cutting fluid. Use long smooth strokes and try not to cut into your guides.

(Fig.8) Remove The guides..
Hopefully the scrap part of your bar has just fallen neatly onto the floor, bench, carpet or dog. You can see from the photo here that despite my words above I DID cut into the guides a bit :(
In the end it didn't effect the finish on the cut edge but did make the guide unusable as a grip clamp again..

(Fig.9) Tart it up a bit..
Even with the best cutting technique, tools and fluids your still likely to end up with a tiny bit of spikey metal on the newly cut surface. Just take this off with a file. I picked some up for about £3 in a local tool shop that did the job. If you cant find a file you can try some wet&dry paper, or even a blunt knife edge.

(Fig.10) Grip up and go ride..
The bars should now be ready to be adorned with grips, brakes and whatever else you want to have on your dashboard. Making the finish nice and straight and smooth is worth it because it means that metal or plastic end-caps sit in/over the bars nice and flush; and the rubber sort don't snag and hole too easily.
 
Safety Note: As mentioned before, do be a little wary on your first ride after you chop the bars. The bike can feel very different in the air and on cornering, especially if you let go of the bars and reach for them again. Seems obvious but we don't want you grazing your knee's (foreheads) now do we?