The only problem with doing what Plankton said for a single stereo song is that it distorts the phase of everything, which isn't exactly a desirable effect.
The best thing to do is make it sound good in the mix, rather than trying to do so after the song is bounced to a single stereo file.
If you're recording multiple instruments on their own individual tracks, do something like this first.
Rhythm guitar 1 - mono track, panned about 50 percent left
Rhythm guitar 2 (separate recording, not duplicated from the first) - mono track, panned about 50 percent right
Lead guitar - Pan center, add stereo or ping pong delay and adjust mix to taste
Drums - if you can record each piece separately, pan them in the mix how the kit is set up. If you can only record the drums to a single track, make sure you're at least using a pair of overheads panned hard left/right on the mixer. If you can only record the drums with a single mic, then they'll just be mono and put in the center. If this is the case, you can pan your lead guitar away from center just to position it off the drums.
Bass - pan center
Lead vocals - pan center
Backup vocals or lead vocal doubling - Record 2 takes, pan each opposite of each other to taste, or maybe around 30 percent. Bring them down in the mix so they don't drown out the lead vocals.
Synths and/or percussion - Pan toward the edges to taste.
EQ:
Make sure to EQ each element so that it sits in its own sonic space. If you put a lot of high frequencies on everything, everything will sound like it's sitting close to the speakers. Conversely, if you don't have a lot of high frequencies on something, they will sit further back from the speakers. Manipulating EQ with this in mind will allow you to create a sense of depth.
Also you should be using EQ so that elements with similar sounds aren't stepping on each other. For bass, you can take out a lot of mid range and carve out a space where the kick drum will sit. Use a spectrum analyzer on the drums to see where the low end energy is at, and cut a few DB from the bass in that region so the kick and the bass don't fight for room and create a muddy mess.
Take out some high freq from things like cymbals and high hats to leave some room for the lead vocals.
Backup vocals don't need as much high freq as the lead, so carve some out of them as well, and they'll also sit further back in the mix.
Guitars don't need a lot of highs either. They like mid range, so make sure your other elements aren't hogging all the mids that your guitars need.
Just be mindful of where you're putting things in the stereo field and how upfront and set back everything should be. If you vary the positioning and EQ of your elements wisely, you'll end up with a wider, more deep sound field.
You can't expect that if you just record all your stuff and leave everything up the middle and don't EQ anything.
Finally, apply compression to tame peaks and raise the lower parts to 'gel' the song together. Experiment with a tiny bit of reverb on certain elements that sit back in the mix. When it's all said and done, you should be most of the way where you want to be. Your goal is to have all the elements clear and distinct, yet positioned in a 3D way for the depth and fullness you're going for.
It will take time and effort to get there, but it will pay off.
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