1. The first thing to remember is that, with all other things equal, a low frequency wave will travel further than a high frequency wave. If you've ever approached an outdoor venue while a band was playing, no doubt the first thing you were probably able to hear is the rumble of the kick drum. So distant sounds aren't very bright (in terms of tonal balance, not intelligence). Use a lowpass filter or roll off the higher frequencies with your favorite EQ to simulate this. The further the sound is intended to be from the listener, the more high end you should roll off.
2. If you are standing far away from a sound source in a reverberant environment, there comes a point where you will be hearing more of the actual reverb than the original signal itself. Thus, if you want to create the illusion of distance, you'll also want to play with the wet/dry mix on your reverb of choice. The 'wetter' the signal, the farther away the sound will appear to be.
For best results, you'll want to utilize both of these techniques simultaneously and in the correct balance. Nothing too difficult to figure out with a little messing around. What about you? Any favorite techniques for putting a sound "far away" from the listener?