3D Mixing

To bring sounds to the front…

Keep the tone fairly bright or use an enhancer.
Don't over-use reverb, and use a brighter reverb for close sounds than for distant ones. Pre-delay can help prevent the reverb from pushing the sound back into the mix.
Give the reverb a wide spread.
For very close sounds, such as whispers, keep the sound very dry and bright.

To push sound backwards…

Make the sounds quieter than the sounds you'd like to appear at your imaginary 'front-of-stage'.
Use high-frequency roll-off to simulate the natural dulling that occurs with distance.
Use narrow panned or even mono reverbs.
Use less bright reverbs than for the 'front-of-stage' sounds.
Use a higher mix of reverb to direct sound and experiment with longer reverb times where you can do so without muddying the mix.
Use echoes that lose HF as they decay.
Don't use an enhancer on distant sounds -- instead, consider patching your enhancer in via a pair of subgroups rather than via the master insert points. This way, you can send just your up-front sounds via the enhancer group, and this will help to produce the contrast you're trying to achieve.