Tempo variations in a score

I have written a cover for a song - voice and piano solo - but due to the varying tempo of the piece, I have added quite a few tempo markings, which doesn't look very tidy or professional on the finished score. Is there any other way of showing/effecting this variation in speed? I have also tried entering a general tempo marking at the start (crotchet = 78 - 85 bpm), but this just plays back the entire piece at 78 bpm!!!