Keep your whole drivetrain; ditch the shifters, cables, and housing. But save one cable. And if the chain keeps wanting to skip off the front chainrings, save the FD in place, but not cabled (use the limit screws for adjustment) as a chain guide:



You can adjust the limit screws as others said, above, but this setup will give you more range -- you can set it up in the range you want or need, and you still get 2-3 gear shifting range with just the barrel adjuster on the derailleur. I left all the chainrings on, too. It wasn't often, but sometimes it was nice to be able to get significantly lower gearing when needed. If you're new to single-speed, something like this is also nice as it will give you a chance to try out different ratios until you get to the one you like best. This bike eventually lost two of the three chainrings and it pretty much stayed in the same gear out back.

Here's another ghetto SS conversion: