I use a couple of techniques, depends on what the source is. For example, with a solo instrument (or even a small ensemble), sometimes it's enough to just pan the lows left and the highs right. Alternatively, for a more subtle, or just different effect, run the signal through (or buss to) a pair of L/R panned reverbs, one filtered low, one filtered high. (Craig Anderton has outlined fancy versions of this EQ-ing-left-and-right method, as have other sources.)
The VC-64 plugin has a couple of presets for "placing" a sound in the L/R image, using its dual compressor/ EQ that I sometimes use as a starting point.
There are plugins--feebies, cheapos, pricey--that do the channel separations for you with a couple of clicks. I use the stereo mixpack set (I think that's the title) from PSP audioware, but there are others. These plugins use paired comb filters and such to achieve a separation, which could probably be done by hand, but the plugins make it easier.
The introduction of the channel tools plugin (sonar 8.5?) made this kind of work a whole lot easier to do inside sonar (in case anyone forgot about channel tools).
Delaying one side by a few (dozen) ms. can work, too. Again, it really depends on the source material. Actually, it REALLY depends on trying things out and listening and picking what sounds to closest to what you want.
AND THEN...monitor the whole faux stereo thing you've put together IN MONO, to make sure, for example, that your one-sided delay tactic, which adds loads of space in stereo, doesn't just smear the whole mono listening experience. Jeff Evans, on these boards, recommends--with good cause--listening to the mono result from ONE speaker