Use an equalizer to boost the key frequencies of the voice-over. At the same time, cut those frequencies from the mixed track. This can make space for the voice-over by reducing the amounts of sound competing in the same range. Try adjustments around 1000 Hz and 2500 Hz to start, and adjust for your situation. Exciter plug-ins and hardware add artificial overtones, and may add presence to your voice-over also.
Process the voice-over through a compressor to even the dynamics and increase emphasis on speech definition frequencies. Start with a compression ratio of about 5:1, adjust the threshold as necessary for your track, and use short attack and release times. Add level if compression reduces the overall volume of the voice-over.
Process the background track through a compressor, but use the voice-over as a side chain trigger. Adjust the compressor so that it reduces the level of the background track when the voice-over is active. For most natural sound, start with a low compression ratio of 2:1 or 3:1, and slower attack and release times. This process is called "ducking" and it can be approximated by using faders to lower the volume of the backing track manually.
Use pitch correction to change the frequency of the voice-over itself. While "flat" has a connotation in singing that is addressed with very specific pitch changes, raising or lowering the pitch of a voice-over may add excitement or depth.